XII. CHÆTOPODA[134]
[134] The following classification of the Chætopoda is adopted in the present section: I. Achæta. (Polygordius). II. Polychæta. { Sedentaria. { Errantia. III. Oligochæta.
Formation of the Germinal Layers.
Most Chætopoda deposit their eggs before development. The Oligochæta lay them in peculiar cocoons or sacks formed by a secretion of the integument. Some marine Polychæta carry them about during their development. Autolytus cornutus has a special sack on the ventral surface in which they are hatched. In Spirorbis Pagenstecheri they develop inside the opercular tentacle, and in Spirorbis spirillum inside the tube of the parent.
A few forms (e.g. Eunice sanguinea, Syllis vivipara, Nereis diversicolor) are viviparous.
Perhaps the most primitive type of Chætopod development so far observed is that of Serpula (Stossich, No. 357)[135]. There is a regular segmentation resulting in the formation of a blastosphere with a central segmentation cavity. An invagination of the normal type now ensues. The blastopore soon narrows to become the permanent anus, while the invaginated hypoblast forms a small prominence with an imperfectly developed lumen, which does not nearly fill up the segmentation cavity (fig. 139 A). The embryo, which has in the meantime become completely covered with cilia, now assumes more or less the form of a cone, at the apex of which is the anus, while the base forms the rudiment of a large præ-oral lobe. The alimentary sack grows forwards and then bends upon itself nearly at right angles, and meets a stomodæal invagination from the ventral side some way from the front end of the body.
[135] The observations of Stossich are not thoroughly satisfactory.
[FIG. 139. TWO STAGES IN THE DEVELOPMENT OF SERPULA. (After Stossich.)
m. mouth; an. anus; al. archenteron.]
The alimentary canal soon differentiates itself into three regions (1) oesophagus, (2) stomach, and (3) intestine. With these changes the larva, which in the meantime becomes hatched, assumes the characters of a typical Annelid larva (fig. 139 B). In front is a large præ-oral lobe, at the sides of which the eye-spots soon appear. The primitive segmentation cavity remains as a wide space between the curved alimentary tract and the body walls, and becomes traversed by muscular fibres passing between the two. The original chorion appears to serve as cuticle, and is perforated by the cilia.
The further changes in this larval form do not present features of general importance. A peculiar vesicle, which in anomalous cases is double, is formed near the anus. If it were shewn to occur widely amongst Chætopoda, it might be perhaps regarded as homologous with the anal vesicles of the Gephyrea.
Serpula is one of the few Chætopoda at present known in which the segmentation is quite regular[136]. In other forms it is more or less unequal. The formation of the germinal layers has been far more fully studied in the Oligochæta than in the Polychæta, and though unfortunately the development is much abbreviated in the former group, they nevertheless have to serve as our type; and unless the contrary is indicated the statements in the remainder of the section apply to the Oligochæta. The segmentation is nearly regular in Lumbricus agricola (Kowalevsky) and results in the formation of a flattened blastosphere, one of the sides of which is hypoblastic and the other epiblastic, the hypoblast cells being easily distinguished from the epiblast cells by their clearer aspect. An invagination takes place, in the course of which the hypoblast becomes enclosed by the epiblast, and a somewhat cylindrical two-layered gastrula is formed. The opening of this gastrula at first extends over the whole of what becomes the ventral surface of the future worm, but gradually narrows to a small pore--the permanent mouth--near the front end. The central cavity of the gastrula is lined by hypoblast cells, but the oral opening, which leads by a narrow passage into the gastric cavity, is lined by epiblast cells.
[136] According to Willemoes-Suhm, Terebellides stroemii is also characterised by a regular segmentation.
The segmentation of Lumbricus trapezoides (Kleinenberg, No. 341), and of Criodrilus (Hatschek, No. 339), is more unequal and more irregular than that of Lumbricus agricola, and there is an invagination which is intermediate between the embolic and epibolic types.
The segmentation of Lumbricus trapezoides is especially remarkable. It is strangely irregular and at one period the segmentation cavity communicates by a pore with the exterior. Before the completion of the gastrula stage the ovum becomes partially divided into two halves, each of which gives rise to a complete embryo. The two embryos are at first united by an epiblast cord which connects their necks (fig. 141 A), but this cord is very early ruptured, and the two embryos then become quite independent. Some of the peculiarities of the segmentation may no doubt be explained by this remarkable embryonic fission.
The gastrula opening in both Lumbricus trapezoides and Criodrilus is placed on the ventral surface, and eventually narrows to form the mouth or possibly (Criodrilus) closes at the position of the mouth. In Lumbricus trapezoides the oral opening is at first lined by hypoblast, and in Criodrilus is bounded anteriorly by three large peculiar epiblast cells, which are believed by Hatschek to assist in absorbing the albuminous fluid in which the eggs are suspended. These large cells are eventually covered by the normal epiblast cells and subsequently disappear. In both these types the hypoblast cells undergo, during their invagination, peculiar changes connected with their nutritive function.
In Euaxes (Kowalevsky) the segmentation is far more unequal than in the other types; a typical epibolic invagination takes place (fig. 140), and the blastopore closes completely along the ventral surface.
[FIG. 140. TRANSVERSE SECTION THROUGH THE OVUM OF EUAXES DURING AN EARLY STAGE OF DEVELOPMENT. (After Kowalevsky.)
ep. epiblast; ms. mesoblastic band; hy. hypoblast.]
In all the oligochætous types, with the exception of Euaxes, where the blastopore closes completely, the blastopore becomes, or coincides with the mouth. In Serpula it is stated (Stossich), as we have seen, to coincide with the anus: a statement which receives confirmation from the similar statements of Willemoes-Suhm (No. 358). It is necessary either to suppose a mistake on the part of Stossich, or that we have in Chætopods a case like that of Gasteropods in which a slit-like blastopore originally extending along the ventral surface may in some forms become reduced to a pore at the oral, or in other forms at the anal extremity.
So far only two germinal layers--the epiblast and the hypoblast--have been spoken of. Before the invagination of the hypoblast is completed the mesoblast makes its appearance in the form of two bands or streaks, extending longitudinally for the whole length of the embryo. These are usually spoken of as germinal streaks, but to avoid the ambiguity of this term they will be spoken of as mesoblastic bands.
Their origin and growth has been most fully studied by Kleinenberg (No. 341) in Lum. trapezoides. They commence in this species shortly before the gastrula stage as two large cells on the surface of the blastoderm, which may be called mesoblasts. These cells lie one on each side of the median line at the hind end of the embryo. They soon travel inwards and become covered by the epiblast (fig. 141 A, m´), while on their inner and anterior side a row of small cells appears (ms). These rows of cells form the commencement of the mesoblastic bands, and in the succeeding stages they extend one on each side of the body (fig. 141 B, ms) till they reach the sides of the mouth. Their forward growth takes place mainly at the expense of the superjacent epiblast cells, but the two mesoblasts at their hinder extremities probably assist in their growth. Each mesoblastic band is at first composed of only a single row of cells, but soon becomes thicker, first of all in front, and becomes composed of two, three or more rows of cells abreast. From the above it is clear that the mesoblastic bands have, in L. trapezoides at any rate, in a large measure an epiblastic origin.
[FIG. 141. THREE SECTIONS ILLUSTRATING THE DEVELOPMENT OF LUMBRICUS TRAPEZOIDES. (After Kleinenberg.)
ms. mesoblastic band; m´. mesoblast; al. archenteron; pp. body cavity. A. Horizontal and longitudinal section of an embryo which is dividing into two embryos at the gastrula stage. It shews the mesoblasts and the mesoblastic bands proceeding from them. B. Transverse section shewing the two widely separated mesoblastic bands. C. Transverse section at a later stage shewing the mesoblastic bands which have approached the ventral line and developed a body cavity pp.]
At first the two bands end in front at the sides of the mouth, but subsequently their front ends grow dorsalwards at the expense of the adjoining epiblast cells, and meet above the mouth, forming in this way a mesoblastic dorsal commissure.
The mesoblastic bands soon travel from the lateral position, which they at first occupy, towards the ventral surface. They do not however meet ventrally for some time, but form two bands, one on each side of the median ventral line (fig. 141 C).
The usual accounts of the origin and growth of the bands differ somewhat from the above. By Kowalevsky (No. 342) and Hatschek (No. 339) they are believed to increase in Lumbricus rubellus and Criodrilus entirely at the expense of the mesoblasts. Kowalevsky moreover holds that in L. rubellus the original mesoblasts spring from the hypoblast. In some forms, e.g. Lumbricus agricola, the mesoblasts are not present.
In Euaxes the origin of the mesoblast bands is somewhat interesting as illustrating the relation of the Chætopod mesoblastic bands to the mesoblast of other forms. To render intelligible the origin of the mesoblast in this form, it is necessary to say a few words about the segmentation.
By a somewhat abnormal process of segmentation the ovum divides into four spheres, of which one is larger than the others, and occupies a position corresponding with the future hind end of the embryo. The three smaller spheres give rise on their dorsal side by a kind of budding to small cells, which become the epiblast; and the epiblast is also partly formed from the hinder large cell in that this cell produces by budding a small cell, which again divides into two. The anterior of the two cells so formed divides still further and becomes incorporated in the epiblast; the posterior only divides into two which form the two mesoblasts. The remainder of the mesoblast is formed by further division of the three smaller of the primitive large spheres, and at first forms a continuous layer between the dorsal cap of epiblast and the four largest cells which, after giving rise to the epiblast and mesoblast, constitute the hypoblast. As the epiblast spreads over the hypoblast the mesoblastic sheet gives way in the middle, and the mesoblast remains as a ridge of cells at the edge of the epiblastic cup. It forms in fact a thickening of the lips of the blastopore. Behind the thickening is completed by the two mesoblasts. The appearance of the mesoblast in section is shewn in fig. 140. As the epiblast accompanied by the mesoblast grows round the hypoblast, the blastopore assumes an oval form, and the mesoblast appears as two bands forming the sides of the oval. The epiblast travels over the hypoblast more rapidly than the mesoblast, so that when the blastopore becomes closed ventrally the mesoblastic bands are still some little way apart on the ventral side.
In Euaxes the mesoblast originates in a manner which is very similar to that in some of the Gasteropoda, e.g. Nassa, vide p. 234, and Vermes, e.g. Bonellia, etc. As mentioned in the chapter on the Mollusca the origin of the mesoblast in Planorbis, p. 227, is very similar to that in Lumbricus.
Hatschek has shewn that in Polygordius the mesoblast arises in fundamentally the same way as in the Oligochæta.
Besides the mesoblast which arises from the mesoblastic bands, there is evidence of the existence of further mesoblast in the larvæ of many Polychæta in the form of muscular fibres which traverse the space between the body wall and the wall of the enteric cavity prior to the formation of the permanent body cavity. These fibres have already been described in the embryo of Serpula, and are probably represented by stellate cells in the cephalic region (præ-oral lobe) of the Oligochæta. These cells are probably of the same nature as the amoeboid cells in the larvæ of Echinodermata, some Mollusca and other types.
The Larval form.
True larval forms are not found in the Oligochæta where the development is abbreviated. They occur however in the majority of the marine Polychæta.
They present a great variety of characters with variously arranged ciliated bands. Most of these forms can be more or less satisfactorily derived from a larval form, like that of Serpula (fig. 139 B) or Polygordius (fig. 142); and the constant recurrence of this form amongst the Chætopoda, combined with the fact that it presents many points of resemblance to the larval forms of many Rotifers, Molluscs, and Gephyreans, seems to point to its being a primitive ancestral form for all these groups.
The important characters of this larval form are (1) the division of the body into a large præ-oral lobe and a relatively small post-oral region containing the greater part of the alimentary tract; (2) the presence of a curved alimentary canal divided into stomodæum (oesophagus), stomach and intestine, and opening by a ventrally placed mouth, and an anus near the hind end of the body. To these may be added the frequent presence of (1) a ganglion at the apex of the præ-oral lobe, (2) a large cavity between the wall of the gut and the skin, which is the remnant of the segmentation cavity, and is usually traversed by muscular strands, of which one connecting the apex of the præ-oral lobe and the stomach or oesophagus is very commonly present (fig. 142).
The arrangement of the ciliated bands presents great variations, though in some instances it is constant through large groups. In Chætopods there is a widely distributed præ-oral ciliated band, which is similarly placed to the ring constantly found in the larvæ of Molluscs, Rotifers, etc. In many of these forms the band is practically double, the opening of the mouth being placed between its two component rings (vide fig. 142). The best introduction to the study of the Chætopod larval forms will be the history of the changes of a typical larval form in becoming converted into the adult.
[FIG. 142. POLYGORDIUS LARVA. (After Hatschek.)
m. mouth; sg. supra-oesophageal ganglion; nph. nephridion; me.p. mesoblastic band; an. anus; ol. stomach.]
For this purpose no better form can be selected than the interesting larva of Polygordius (vide Agassiz, No. 332, Schneider, No. 352, and Hatschek, No. 339), which was first discovered by Lovén, and believed by him to be the larva of an ordinary Chætopod. Its true nature was determined by Schneider.
At a very young stage the larva has the form (fig. 142) of a flattened sphere, with a small conical knob at the posterior extremity.
At the equator are situated two parallel ciliated bands[137], between which lies the ventrally placed mouth (m). The more conspicuous ciliated band is formed of a double row of cilia, and is situated in front of the mouth. The thinner ciliated band behind the mouth appears to be absent in the American species.
[137] These two rings are at first (Hatschek) not quite closed dorsally, calling to mind the early condition of the Echinoderm larvæ with a præ-oral and post-oral ciliated area.
The mouth leads into an oesophagus, and this into a globular stomach (ol), which is continuous with a rectum terminating by an anus (an) placed at the hind end of the posterior conical knob. The whole alimentary tract is ciliated. In the American form of larva there is a ring of cilia round the anus, which is developed at a somewhat later stage in the form observed by Hatschek.
[FIG. 143 POLYGORDIUS LARVA. (From Alex. Agassiz.)]
The position of the ciliated bands and the alimentary tract enables us to divide the embryo into three regions: a præ-oral region bounded by the anterior ciliated band, a gastric region in which the embryonic stomach is situated, and an abdominal region formed of the posterior conical portion, which by its subsequent elongation gives rise to the whole segmented portion of the future Polygordius.
At the front end of the præ-oral lobe is situated the early formed supra-oesophageal ganglion (sg) (first noticed by Agassiz) in connection with which is a pair of eyes, and a ramified system of nerves. The ganglion is marked externally by a crown of cilia.
[FIG. 144. POLYGORDIUS LARVA. (From Alex. Agassiz.)]
The larval epidermis bears a delicate cuticula, and is separated by a considerable interval from the walls of the alimentary tract. The space between the two represents a provisional body cavity, which is eventually replaced by the permanent body cavity formed between the two layers of the mesoblast. It is doubtful when the replacement takes place in the head. It probably does so very early. The mesoblast is present in the usual form of two bands (me.p) (germinal streaks), which are anteriorly continued into two muscular bands which pass through the embryonic body cavity to the front end of the præ-oral lobe. Another pair of contractile bands passes from the same region of the præ-oral lobe to the oesophagus.
There is no trace of the ventral nerve cord. The most remarkable organ of the larva is a paired excretory organ (nph) discovered by Hatschek. This is a ciliated canal with at first one and subsequently several funnel-shaped openings into the body cavity in front and an external opening behind. It is situated immediately anterior to the lateral band of mesoblast, and is parallel with, and dorsal to, the contractile band which passes off from this. It occupies therefore a position in front of the segmented region of the adult Polygordius.
[FIG. 145. POLYGORDIUS LARVA. (From Alex. Agassiz.)]
The changes by which this peculiar larval form reaches the adult condition will be easily gathered from an inspection of figs. 143-148. They consist essentially in the elongation of what has been termed the abdominal region of the body, and the appearance of a segmentation in the mesoblast; the segments being formed from before backwards, and each fresh segment being interpolated between the anus-bearing end of the body and the last segment.
As the hind portion of the body becomes elongated the stomach extends into it, and gives rise to the mesenteron of the adult (figs. 143, 144, and 145). For a long time the anterior spherical dilated portion of the larva remains very large, consisting of a præ-oral lobe and a post-oral section. The two together may be regarded as constituting the head.
At a comparatively late stage a pair of tentacles arises from the front end of the præ-oral lobe (fig. 146), and finally the head becomes relatively reduced as compared with the body, and gives rise to the simple head of the fully formed worm (fig. 148). The two ciliated bands disappear, the posterior vanishing first. The ciliated band at the hind end of the body also atrophies; and just in front of it the ring of wart-like prominences used by the adult to attach itself becomes developed.
[FIG. 146. POLYGORDIUS LARVA. (From Alex. Agassiz.)]
At the sides of the head there is formed a pair of ciliated pits, also found by Hatschek in the embryo of Criodrilus, and characteristic of many Chætopod larvæ, but persistent in the adult Polygordius, Saccocirrus, Polyophthalmus, etc. They are perhaps the same structures as the ciliated pits in Nemertines.
[FIG. 147. POLYGORDIUS LARVA. (From Alex. Agassiz.)]
During the external changes above described, by which the adult form of Polygordius is reached, a series of internal changes also takes place which are for the most part the same as in other Chætopoda; and do not require a detailed description. The nervous[138] and muscular systems have precisely the normal development. The division of the mesoblast into somites is not externally indicated. The organs most worthy of notice are the excretory organs.
[138] The structure of the ventral cord in the adult requires further elucidation.
The essential points in the above development of Polygordius are (1) the gradual elongation and corresponding segmentation of the post-cephalic part of the body; and (2) the relative reduction in size of the præ-oral lobe and its conversion together with the oral region into the head; (3) the atrophy of the ciliated bands. The conversion of the larva into the adult takes place in fact by the intercalation of a segmented region between a large mouth-bearing portion of the primitive body and a small anus-bearing portion[139].
[139] For Semper's view as to the intercalation of segments in the cephalic region, vide note on p. 333.
[FIG. 148. POLYGORDIUS LARVA. (From Alex. Agassiz.)]
The general mode of development of Chætopod larvæ is similar to the above except in details, which are however no doubt often of great importance. The history of the larvæ may be conveniently treated under three heads. (1) The form of the primitive unsegmented larva; (2) the arrangement of the cilia on the unsegmented larva, and on the larva at later stages; (3) the character of the metamorphosis and the development of the permanent external organs.
A larva similar to the Polygordius larva with a greatly developed præ-oral lobe is widely distributed amongst the Annelids.
An almost identical form is that of Nepthys scolopendroides (Claparède and Metschnikoff, No. 336); that of Phyllodoce (fig. 149) is also very similar, and that of Saccocirrus (Metsch. and Clap. No. 336, Pl. XIII. fig. 1), a very primitive form most nearly related to Polygordius, clearly belongs to the same type. Many other larval forms, such as that of Spio fuliginosus (Metsch. and Clap. No. 336), Terebella, Nerine, etc., also closely approach this form.
[FIG. 149. LARVA OF PHYLLODOCE. (From Alex. Agassiz.)]
Other really similar forms at first sight appear very different, but this is mainly owing to the fact that their præ-oral lobe never attains a considerable development. Its smallness, though obviously of no deep morphological significance, at once produces a very different appearance in a larva.
A good example of a larval form with a small præ-oral lobe is afforded by Capitella, which is figured by Clap. and Metsch. (No. 336, Pl. XVII. fig. 2). The imperfect development of the præ-oral lobe is also generally characteristic of the Oligochæta. The persistence of a relatively large præ-oral lobe for so long a time as in Polygordius is very unusual.
The arrangement of the cilia in Chætopod larvæ has been employed as a means of classifying them. Although a classification so framed has no morphological value, yet the terms themselves which have been invented are convenient. The terms most usually employed are Atrochæ, Monotrochæ, Telotrochæ, Polytrochæ, Mesotrochæ. The polytrochæ may again be subdivided into Polytrochæ proper, Nototrochæ, Gasterotrochæ, and Amphitrochæ.
The atrochæ contain forms (fig. 139) in which the larva is at first coated by an uniform covering of cilia, which, though it may subsequently disappear from certain areas, does not break up into a series of definite bands.
The monotrochæ or cephalotrochæ are larvæ in which only a single præ-oral ring is developed (fig. 150 B).
[FIG. 150. TWO CHÆTOPOD LARVÆ. (From Gegenbaur.)
o. mouth; i. intestine; a. anus; v. præ-oral ciliated band; w. peri-anal ciliated band.]
In the telotrochæ there is present a præ-oral and a post-oral, i.e. peri-anal ring (fig. 150 A); the latter sometimes having the form of a peri-anal patch.
The polytrochæ are segmented larvæ with perfect or imperfect rings of cilia on the segments of the body--usually one ring to each segment--between the two characteristic telotrochal rings. When these rings are complete the larvæ are polytrochæ proper, when they are only half rings they are either nototrochæ or gasterotrochæ. Sometimes there are both dorsal and ventral half rings which do not however correspond, such forms constitute the amphitrochæ.
In the mesotrochæ one or two rings are present in the middle of the body, and the characteristic telotrochal rings are absent. Larvæ do not necessarily continue to belong to the same group at all ages. A larva may commence as a monotrochal form and then become telotrochal and from this pass into a polytrochal condition, etc.
The atrochal forms are to be regarded as larvæ which never pass beyond the primitive stage of uniform ciliation, which in other instances may precede that of definite rings. They usually lose their cilia early, as in the cases of Serpula and other larvæ described below.
The atrochal larvæ are not common. The following history of an Eunicidan larva (probably Lumbriconereis) from Claparède and Metschnikoff (No. 336) will illustrate their general history.
In the earliest stage noticed the larva has a spherical form, the præ-oral lobe not being very well marked. In the interior is a globular digestive tract. The cilia form a broad central band leaving free a narrow space at the apex of the præ-oral lobe, and also a circumanal space. At the apex of the præ-oral lobe is placed a bunch of long cilia, and a patch of cilia also marks out the anal area.
As the larva grows older it becomes elongated, and the anterior bunch of cilia is absorbed. The alimentary canal divides itself into pharynx and intestine. The former opens (?) by the mouth in the middle of the central band of cilia, the latter in the anal patch. The setæ indicating the segmentation are formed successively in the posterior ring-like area free from cilia. The cilia disappear after the formation of two segments.
In Lumbricus, the embryo of which ought perhaps to be grouped with the atrochæ, the cilia (Kleinenberg) cover a ventral tract of epiblast between the two mesoblastic cords, and are continued anteriorly to form a circle round the mouth.
The monotrochal larvæ are provided only with the important præ-oral ciliated ring before mentioned. In the majority of cases they are transitional forms destined very shortly to become telotrochal, and in such instances they usually have a more or less spherical body which is nearly divided into two equal halves by a ciliated ring. In some few instances, such as Polynoe, Dasychone, etc., the monotrochal characters are not lost till the larval cilia are exuviated.
The telotrochal forms (of which examples are shewn in figs. 144, 150, etc.) may (1) start as monotrochal; or (2) from the first have a telotrochal character; or (3) be derived from atrochal forms. The last mode of origin probably represents the ancestral one.
Their mode of development is well illustrated by the case of Terebella nebulosa (vide Milne-Edwards, No. 347). The embryo is at first a nearly spherical ciliated mass. One end slightly elongates and becomes free from cilia, and, acquiring dorsally two eye-spots, constitutes a præ-oral lobe. The elongation continues at the opposite end, and near this is formed a narrow area free from cilia. The larva now has the same characters as the atrochal Eunicidan larva described above. It consists of a non-ciliated præ-oral lobe, followed by a wide ciliated band, behind which is a ring-like area free from cilia; and behind this again a peri-anal patch of cilia. The ring-like area free from cilia is, as in the Eunicidan larva, the region which becomes segmented. It soon becomes longer, and is then divided into two segments; a third and fourth etc. non-ciliated segment becomes successively interposed immediately in front of the peri-anal patch; and, after a certain number of segments have become formed, there appear on some of the hinder of them short tubercles, provided with single setæ (the notopodia), which are formed from before backwards, like the segments.
The mouth, anus, and intestine become in the meantime clearly visible. The mouth is on the posterior side of the ciliated band, and the anus in the centre of the peri-anal patch.
The ciliated band in front now becomes contracted and provided with long cilia. It passes below completely in front of the mouth, and constitutes, in fact, a well-marked præ-oral ring, while the cilia behind constitute an equally marked peri-anal ring. The larva has in fact now acquired all the characters of a true telotrochal form.
Only a comparatively small number of Chætopod larvæ remain permanently telotrochal. Of these Terebella nebulosa, already cited (though not Terebella conchilega), is one; Polygordius, Saccocirrus and Capitella are other examples of the same, though in the latter form the whole ventral surface becomes ciliated.
The majority of the originally telotrochal forms become polytrochal.
In most cases the ciliated rings or half rings of the polytrochal forms are placed at equal distances, one for each segment. They are especially prominent in surface-swimming larvæ, and are in rare cases preserved in the adult. In some instances (e.g. Nerine and Spio) the ventral half rings, instead of being segmentally arranged, are somewhat irregularly distributed amongst the segments, so that there does not seem to be a necessary correspondence between the ciliated rings and the segments. This is further shewn by the fact that the ciliated rings are not precursors of the true segmentation, but are developed after the establishment of the segments, and thus seem rather to be secondarily adapted to the segments than primarily indicative of them.
In most Polytrochæ the rings are incomplete, so that they fall under the category of Nototrochæ or Gasterotrochæ.
The larva of Odontosyllis is an example of the former, and that of Magelona of the latter. The larvæ of Nerine and Spio, already quoted as examples of an unsegmented arrangement of the ventral ciliated half rings, are both amphitrochal forms.
As an example of a polytrochal form with complete ciliated rings Ophryotrocha puerilis may be cited. This form, discovered by Claparède and Metschnikoff, develops a complete ciliated ring on each segment: and the præ-oral ring, though at first single, becomes at a later period divided into two. This form is further exceptional in that the ciliated rings are persistent in the adult.
The unimportance of the character of the rings in the polytrochal forms is shewn by such facts as the absence of these rings in Terebella nebulosa and the presence of dorsal half rings in Terebella conchilega.
The mesotrochal forms are the rarest of Chætopod larvæ, and would seem to be confined to the Chætopteridæ.
Their most striking character is the presence of one or two complete ciliated rings which girth the body between the mouth and anus. The whole body is further covered with short cilia. The anus has a distinct dorsal situation, while on its ventral side there projects backwards a peculiar papilla.
The total absence of the typical præ-oral and of the peri-anal bands separates the mesotrochal larvæ very sharply from all the previous types.
A characteristic of many Chætopod larvæ is the presence of a bunch of cilia or a single flagellum at the apex of the præ-oral lobe. The presence of such a structure is characteristic of the larval forms of many other groups, Turbellarians, Nemertines, Molluscs, etc.
In the preceding section the mode of multiplication of the segments has already been sufficiently described[140].
[140] It has been insisted by Semper (No. 355) that certain of the anterior segments, belonging to what he regards as the head region in opposition to the trunk, become interpolated between the trunk and the head. The general evidence, founded on observations of budding, which he brings forward, cannot be discussed here. But the special instance which he cites (founded on Milne-Edwards's (No. 347) observations) of the interpolation of the head segments, bearing the gills, in Terebella appears to me quite unjustified from Milne-Edwards's own statements; and is clearly shewn to be unfounded by the careful observations of Claparède on Ter. conchilega, where the segments in question are demonstrated to be present from the first.
[FIG. 151. LARVA OF PHYLLODOCE FROM THE VENTRAL SIDE. (From Alex. Agassiz.)]
Apart from the formation of the segments the larval metamorphosis consists in the atrophy of the provisional ciliated rings and other provisional organs, and in the acquirement of the organs of the adult.
The great variations in the nature of the Chætopod appendages render it impossible to treat this part of the developmental history of the Chætopoda in a systematic way.
The mode of development of the appendages is not constant, so that it is difficult to draw conclusions as to the primitive form from which the existing types of appendages are derived.
In a large number of cases the primitive rudiments of the feet exhibit no indication of a division into notopodium and neuropodium; while in other instances (e.g. Terebella and Nerine, fig. 152) the notopodium is first developed, and subsequently the neuropodium quite independently.
In many cases the setæ appear before there are any other visible rudiments of the feet (e.g. Lumbriconereis); while in other cases the reverse holds good. The gills are usually the last parts to appear.
[FIG. 152. LARVA OF NERINE, WITH PROVISIONAL SETÆ. (From Alex. Agassiz.)]
Not only does the mode of development of the feet differ greatly in different types, but also the period. The appearance of setæ may afford the first external indication of segmentation, or the rudiments of the feet may not appear till a large number of segments are definitely established.
A very considerable number of Chætopod larvæ are provided with very long provisional setæ (figs. 152 and 153). These setæ are usually placed at the sides of the anterior part of the body, immediately behind the head, and also sometimes on the posterior parts of the body. In some instances (e.g. fig. 153) they form the only appendages of the trunk. Alex. Agassiz has pointed out that setæ of this kind, though not found in existing Chætopods, are characteristic of the fossil forms. Setæ of this kind are found in chætopod-like larvæ of some Brachiopods (Argiope, fig. 136).
[FIG. 153. EMBRYO CHÆTOPOD WITH PROVISIONAL SETÆ. (From Agassiz.)]
It is tempting to suppose that the long provisional bristles springing from the oral region are the setiform appendages handed down from the unsegmented ancestors of the existing Chætopod forms. Claparède has divided Chætopod larvæ into two great groups of Metachætæ and Perennichætæ, according as they possess or are without provisional setæ.
With reference to the head and its appendages it has already been stated that the head is primarily formed of the præ-oral lobe and of the peristomial region.
The embryological facts are opposed to the view that the præ-oral region either represents a segment or is composed of segments equivalent to those of the trunk. The embryonic peristomial region may, on the other hand, be regarded as in a certain sense the first segment. Its exact relations to the succeeding segments become frequently more or less modified in the adult. The præ-oral region is in most larvæ bounded behind by the ciliated ring already described. On the dorsal part of the præ-oral lobe in front of this ring are placed the eyes, and from it there may spring a variable number of processes which form antennæ or cephalic tentacles. The number and position of these latter are very variable. They appear as simple processes, sometimes arising in pairs, and at other times alternating on the two sides. There is frequently a median unpaired tentacle.
The development of the median tentacle in Terebella, where there is in the adult a great number of similar tentacles, is sufficiently remarkable to deserve special notice; vide Milne-Edwards, Claparède, etc. It arises long before any of the other tentacles as a single anterior prolongation of the præ-oral lobe containing a parenchymatous cavity, which communicates freely with the general perivisceral cavity. It soon becomes partially constricted off at its base from the procephalic lobe, but continues to grow till it becomes fully half as long as the remainder of the body. A very characteristic figure of the larva at this stage is given by Claparède and Metschnikoff, Pl. XVII., Fig. 1 E. It now strikingly resembles the larval proboscis of Balanoglossus, and it is not easy to avoid the conclusion that they are homologous structures.
Another peculiar cephalic structure which deserves notice is the gill apparatus of the Serpulidæ.
In Dasychone (Sabella) the gill apparatus arises (Claparède and Metschnikoff, No. 336) as a pair of membranous wing-like organs on the dorsal side of the præ-oral lobe immediately in front of the ciliated ring. Each subsequently becomes divided into two rays, and new rays then begin to sprout on the ventral side of the two pairs already present. A cartilaginous axis soon becomes formed in these rays, and after this is formed fresh rays sprout irregularly from the cartilaginous skeleton.
[FIG. 154. LARVA OF SPIRORBIS. (From Alex. Agassiz.)
The first odd tentacle (t) is shewn on the right side. Behind the præ-oral ciliated ring is the large collar.]
In Spirorbis spirillum as observed by Alex. Agassiz, the right gill-tentacle (fig. 154, t) first appears, and then the left, and subsequently the odd opercular tentacle which covers the right original tentacle. The third and fourth tentacles are formed successively on the two sides, and rapidly become branched in the succeeding stages.
With reference to the sense organs it may be noted that the eyes, or at any rate the cephalic pigment spots, are generally more numerous in the embryo than in the adult, and that they are usually present in the larvæ of the Sedentaria, though absent in the adults of these forms. The Sedentaria thus pass through a larval stage in which they resemble the Errantia.
Paired auditory vesicles of a provisional character have been found on the ventral side of the body, in the fourth segment behind the mouth, in the larva of Terebella conchilega (Claparède).
Mitraria. A peculiar larval Chætopod form known as Mitraria, the metamorphosis of which was first worked out by Metschnikoff, deserves a special notice.
This form (fig. 155 A) in spite of its remarkable appearance can easily be reduced to the normal type of larva.
The mouth (m) and anus (an) (fig. 155 A) are closely approximated, and situated within a vestibule the edge of which is lined by a simple or lobed ciliated ring. The shape of the body is somewhat conical. The cavity of the vestibule forms the base of the cone, and at the apex is placed a ciliated patch (sg). A pair of lobes (br) bear provisional setæ. The alimentary canal is formed of the three normal parts, oesophagus, stomach, and intestine.
[FIG. 155. TWO STAGES IN THE DEVELOPMENT OF MITRARIA. (After Metschnikoff.)
m. mouth; an. anus; sg. supra-oesophageal ganglion; br. provisional bristles; pr.b. præ-oral ciliated band.]
To compare this larva with an ordinary Chætopod larva one must suppose that the alimentary canal is abnormally bent, so that the post-oral ventral surface is reduced to the small space between the mouth and the anus. The ciliated band surrounding the vestibule is merely the usual præ-oral band, borne on the very much extended edge of the præ-oral lobe. The apex of the larva is the front end of the præ-oral lobe with the usual ciliated patch. The two lobes with provisional bristles are really dorsal and not posterior.
The correctness of the above interpretation is clearly shewn by the metamorphosis.
The first change consists in the pushing in of a fold of skin, between the mouth and anus, towards the intestine, which at the same time rapidly elongates, and forms the axis of a conical projection, which thereupon becomes segmented and is thereby shewn to be the rudiment of the trunk (fig. 155 B). On the elongation of the trunk in this way the præ-oral lobe and its ciliated ring assume an appearance not very dissimilar to the same structures in Polygordius. At the ciliated apex of the præ-oral lobe a paired thickening of epiblast gives rise to the supra-oesophageal ganglia (sg). In the further metamorphosis, the præ-oral lobe and its ciliated ring gradually become reduced, and finally atrophy in the normal way, while the trunk elongates and acquires setæ. The dorsally situated processes with provisional setæ last for some time, but finally disappear. The young worm then develops a tube and shews itself as a normal tubicolous Chætopod.
Formation of Organs.
Except in the case of a few organs our knowledge of the formation of the organs in the Chætopoda is derived from investigations on the Oligochæta.
The embryo of the Oligochæta has a more or less spherical form, but it soon elongates, and becoming segmented acquires a distinct vermiform character. The ventral surface is however for a considerable time markedly convex as compared to the dorsal.
The ventrally placed mouth is surrounded by a well-marked lip, and in front of it is placed a small præ-oral lobe.
[FIG. 156. SECTION THROUGH THE HEAD OF A YOUNG EMBRYO OF LUMBRICUS TRAPEZOIDES. (After Kleinenberg.)
c.g. cephalic ganglion; cc. cephalic portion of the body cavity; x. oesophagus.]
The epiblast. The epiblast cells at the commencement of the gastrula stage become much flattened, and on the completion of the invagination form an investment of flattened cells, only thickened in the neighbourhood of the mesoblastic bands (fig. 141 B and C). In the Polychæta at any rate the statements of several investigators would seem to indicate that the cuticle is derived from the chorion. It is difficult to accept this conclusion, but it deserves further investigation.
Nervous system. The most important organ derived from the epiblast is the nervous system; the origin of which from this layer was first established by Kowalevsky (No. 342).
[FIG. 157. SECTION THROUGH PART OF THE VENTRAL WALL OF THE TRUNK OF AN EMBRYO OF LUMBRICUS TRAPEZOIDES. (After Kleinenberg.)
m. longitudinal muscles; so. somatic mesoblast; sp. splanchnic mesoblast; hy. hypoblast; Vg. ventral nerve cord; vv. ventral vessel.]
It arises[141] (Kleinenberg, No. 341) from two at first quite distinct structures, viz. (1) the supra-oesophageal rudiment and (2) the rudiment of the ventral cord. The former of these takes its origin as an unpaired dorsal thickening of the epiblast at the front end of the head (fig. 156, c.g.), which sends two prolongations downwards and backwards to meet the ventral cord. The latter arises as two independent thickenings of the epiblast, one on each side of the ventral furrow (fig. 157, Vg). These soon unite underneath the furrow, in the median line, and after being differentiated into segmentally arranged ganglionic and interganglionic regions become separated from the epiblast. Both the supra-oesophageal and ventral cord become surrounded by a layer of somatic mesoblast. The junction between the two parts of the central nervous system takes place comparatively late.
[141] For further details, vide general chapter on Nervous System.
The mesoblast. It is to Kowalevsky (No. 342) and Kleinenberg (No. 341) that we mainly owe our knowledge of the history of the mesoblast. The fundamental processes which take place are (1) the splitting of the mesoblast into splanchnic and somatic layers with the body cavity between them, (2) the transverse division of the mesoblast of the trunk into distinct somites.
The former process commences in the cephalic mesoblastic commissure, where it results in the formation of a pair of cavities each with a thin somatic and thick splanchnic layer (fig. 156, cc); and thence extends gradually backwards into the trunk (fig. 141 C, pp). In the trunk however the division into somites precedes the horizontal splitting of the mesoblast. The former process commences when the mesoblastic bands form widish columns quite separate from each other. These columns become broken up successively from before backwards into somewhat cubical bodies, in the centre of which a cavity soon appears. The cavity in each somite is obviously bounded by four walls, (1) an outer, the somatic, which is the thickest; (2) an inner, the splanchnic; and (3, 4) an anterior and posterior. The adjoining anterior and posterior walls of successive somites unite together to form the transverse dissepiments of the adult, which subsequently become very thin and are perforated in numerous places, thus placing in communication the separate compartments of the body cavity. The somites, though at first confined to a small area on the ventral side, gradually extend so as to meet their fellows above and below and form complete rings (fig. 157) of which the splanchnic layer (sp) attaches itself to the enteric wall and the somatic (so) to the epiblast. In Polygordius and probably also Saccocirrus and other forms the cavities of the somites of the two sides do not coalesce; and the walls which separate them constitute dorsal and ventral mesenteries. The two cavities in the cephalic commissure unite dorsally, but ventrally open into the first somite of the trunk.
The mesoblastic masses of the head are probably not to be regarded as forming a pair of somites equivalent to those in the trunk, but as forming the mesoblastic part of the præ-oral lobe, of which so much has been said in the preceding pages. Kleinenberg's observations are however of great importance as shewing that the cephalic cavities are simply an anterior part of the true body cavity.
The splanchnic layer of the head cavity gives rise to the musculature of the oesophagus.
The somatic layer of the trunk somites becomes converted into the musculature of the body wall and the external peritoneal layer of body cavity. The first part of the muscular system to be definitely formed is the ventral band of longitudinal muscles which arises on each side of the nervous system in contact with the epidermis (fig. 157, m). How the circular muscles become subsequently formed outside these muscles has not been made out.
The splanchnic layer of the trunk somites gives rise to the muscular and connective-tissue wall of the mesenteron, and also to the walls of the vascular trunks. The ventral vessel is first formed (Kowalevsky) as a solid mass of cells which subsequently becomes hollowed out. The dorsal vessel in Lumbricus and Criodrilus is stated by Kowalevsky and Vejdovsky to be formed by the coalescence of two lateral vessels; a peculiarity which is probably to be explained by the late extension of the mesoblast into the dorsal region.
The layer from which the sacks for the setæ and the segmental organs spring is still doubtful. The sacks for the setæ are believed by Kowalevsky (No. 342) to be epiblastic invaginations, but are stated by Hatschek (No. 339) to be mesoblastic products. For the development of the segmental organs the reader is referred to the chapter on the excretory system.
In marine Polychæta the generative organs are no doubt mesoblastic products, as they usually spring from the peritoneal epithelium, especially the parts of it covering the vascular trunks.
The Alimentary Canal. In Lumbricus the enteric cavity is formed during the gastrula stage. In Criodrilus the hypoblast has at first no lumen, but this becomes very soon established. In Euaxes on the other hand, where there is a true epibolic gastrula, the mesenteron is at first represented by a solid mass of yolk (i.e. hypoblast) cells. As the central amongst these become absorbed a cavity is formed. The protoplasm of the yolk cells which line this cavity unites into a continuous polynuclear layer containing at intervals masses of yolk. These masses become gradually absorbed, and the protoplasmic wall of the mesenteron then breaks up into a cylindrical glandular epithelium similar to that of the other types.
In Lumbricus and Criodrilus the blastopore remains as the mouth, but in Euaxes a new mouth or rather stomodæum is formed by an epiblastic invagination between the front end of the two mesoblastic bands. This epiblastic invagination forms the permanent oesophagus; and in Lumbricus trapezoides and Criodrilus, where the oral opening is at first lined by hypoblast, the epiblast soon becomes inflected so as to line the oesophageal region. The splanchnic mesoblast of the cephalic region subsequently invests the oesophagus, and some of its cells penetrating between the adjoining epiblast cells give rise to a thick wall for this part of the alimentary tract; the original epiblast cells being reduced to a thin membrane. This mesoblastic wall is sharply separated from the muscular wall outside, which is also formed of splanchnic mesoblast.
The anus is a late formation.
Alternations of generations.
Amongst Chætopoda a considerable number of forms exhibit the phenomenon of alternations of generations, which in the same general way as in the case of the Coelenterata, is secondarily caused by budding or fission.
The process of fission essentially consists in the division of a parent form into two zooids by the formation of a zone of fission between two old rings, which becomes differentiated (1) into an anal zone in front which forms the anal region of the anterior zooid, and (2) into a cephalic zone behind which forms the head and some of the succeeding segments of the posterior zooid. The anal zone is capable, by growth and successive segmentation, of giving rise to an indefinite number of fresh segments.
In Protula Dysteri, as shewn by Huxley, there is a simple fission into two in the way described. Sexual reproduction does not take place at the same time as reproduction by fission, but both zooids produced are quite similar and multiply sexually.
In the freshwater forms Nais and Chætogaster a more or less similar phenomenon takes place. By a continual process of growth in the anal zones, and the formation of fresh zones of fission whenever four or five segments are added in front of an anal zone, complicated chains of adhering zooids are produced, each with a small number of segments. As long as the process of fission continues sexual products are not developed, but eventually the chains break up, the individuals derived from them cease to go on budding, and, after developing a considerably greater number of segments than in the asexual state, reproduce themselves sexually. The forms developed from the ovum then repeat again the phenomenon of budding, etc., and so the cycle is continued[142].
[142] Reproduction by budding and formation of the sexual products to some extent overlap.
The phenomena so far can hardly be described as cases of alternation of generations. The process is however in certain types further differentiated. In Syllis (Quatrefages) fission takes place, the parent form dividing into two, of which only the posterior after its detachment develops sexual organs. The anterior asexual zooid continues to produce fresh sexual zooids by fission. In Myrianida also, where a chain of zooids is formed, the sexual elements seem to be confined to the individuals produced by budding.
The cases of Syllis and Myrianida seem to be genuine examples of alternations of generations, but a still better instance is afforded by Autolytus (Krohn, No. 343, and Agassiz, No. 333).
In Autolytus cornutus the parent stock, produced directly from the egg, acquires about 40-45 segments, and then gives rise by fission, with the production of a zone of fission between about the 13th and 14th rings, to a fresh zooid behind. This after becoming fully developed into either a male or a female is detached from the parent stock, from which it very markedly differs. The males and females are moreover very different from each other. In the female zooid the eggs are carried into a kind of pouch where they undergo their development and give rise to asexual parent stocks. After the young are hatched the female dies. The asexual stock, after budding off one asexual zooid, elongates again and buds off a second zooid. It never develops generative organs.
The life history of some species of the genus Nereis presents certain very striking peculiarities which have not yet been completely elucidated.
As was first shewn by Malmgren asexual examples of various species of Nereis may acquire the characters of Heteronereis and become sexually mature.
The metamorphosis of Nereis Dumerilii has been investigated by Claparède, who has arrived at certain very remarkable conclusions. He finds that there are two distinct sexual generations of the Nereis form of this species, and two distinct sexual generations of the Heteronereis form.
One sexual Nereis, characterized by its small size, is dioecious, the other discovered by Metschnikoff is hermaphrodite.
Of the Heteronereis sexual forms, both are dioecious, one is small, and swims on the surface, the other is larger and lives at the bottom.
How these various generations are mutually related has not been made out; but Claparède traced the passage of large asexual examples of the Nereis form into the large Heteronereis form.
BIBLIOGRAPHY.
(332) Alex. Agassiz. "On the young stages of a few Annelids." Annals Lyceum Nat. Hist. of New York, Vol. VIII. 1866.
(333) Alex. Agassiz. "On the embryology of Autolytus cornutus and alternations of generations, etc." Boston Journal of Nat. History, Vol. VII. 1859-63.
(334) W. Busch. Beobachtungen ü. Anat. u. Entwick. einiger wirbelloser Seethiere, 1851.
(335) Ed. Claparède. Beobachtungen ü. Anat. u. Entwick. wirbelloser Thiere an d. Küste von Normandie. Leipzig, 1863.
(336) Ed. Claparède u. E. Metschnikoff. "Beiträge z. Kenntniss üb. Entwicklungsgeschichte d. Chætopoden." Zeit. f. wiss. Zool. Vol. XIX. 1869.
(337) E. Grube. Untersuchungen üb. Entwicklung d. Anneliden. Königsberg, 1844.
(338) B. Hatschek. "Beiträge z. Entwick. u. Morphol. d. Anneliden." Sitz. d. k. Akad. Wiss. Wien, Vol. LXXIV. 1876.
(339) B. Hatschek. "Studien über Entwicklungsgeschichte der Anneliden." Arbeiten aus d. zoologischen Institute d. Universität Wien. Von C. Claus. Heft III. 1878.
(340) Th. H. Huxley. "On hermaphrodite and fissiparous species of tubicolar Annelidæ (Protula)." Edinburgh New Phil. Journal, Vol. I. 1855.
(341) N. Kleinenberg. "The development of the earthworm Lumbricus trapezoides." Quart. J. of Micr. Science, Vol. XIX. 1879. Sullo sviluppo del Lumbricus trapezoides. Napoli, 1878.
(342) A. Kowalevsky. "Embryologische Studien an Würmern u. Arthropoden." Mém. Acad. Pétersbourg, Series VII. Vol. XVI. 1871.
(343) A. Krohn. "Ueber die Erscheinungen bei d. Fortpflanzung von Syllis prolifera u. Autolytus prolifer." Archiv f. Naturgesch. 1852.
(344) R. Leuckart. "Ueb. d. Jugendzustände ein. Anneliden, etc." Archiv f. Naturgesch. 1855.
(345) S. Lovén. "Beobachtungen ü. die Metamorphose von Anneliden." Weigmann's Archiv, 1842.
(346) E. Metschnikoff. "Ueber die Metamorphose einiger Seethiere (Mitraria)." Zeit. f. wiss. Zool. Vol. XXI. 1871.
(347) M. Milne-Edwards. "Recherches zoologiques, etc." Ann. Scie. Natur. III. Série, Vol. III. 1845.
(348) J. Müller. "Ueb. d. Jugendzustände einiger Seethiere." Monats. d. k. Akad. Wiss. Berlin, 1851.
(349) Max Müller. "Ueber d. weit. Entwick. von Mesotrocha sexoculata." Müller's Archiv, 1855.
(350) Quatrefages. "Mémoire s. l'embryogénie des Annelides." Ann. Scie. Natur. III. Série, Vol. X. 1848.
(351) M. Sars. "Zur Entwicklung d. Anneliden." Archiv f. Naturgeschichte, Vol. XI. 1845.
(352) A. Schneider. "Ueber Bau u. Entwicklung von Polygordius." Müller's Archiv, 1868.
(353) A. Schneider. "Entwicklung u. system. Stell. d. Bryozoen u. Gephyreen (Mitraria)." Archiv f. mikr. Anat. Vol. V. 1869. (354) M. Schultze. Ueb. die Entwicklung von Arenicola piscatorum u. anderer Kiemenwürmer. Halle, 1856.
(355) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere." Arbeiten a. d. zool.-zoot. Instit. Würzburg, Vol. III. 1876-7.
(356) C. Semper. "Beiträge z. Biologie d. Oligochæten." Arbeiten a. d. zool.-zoot. Instit. Würzburg, Vol. IV. 1877-8.
(357) M. Stossich. "Beiträge zur Entwicklung d. Chætopoden." Sitz. d. k. k. Akad. Wiss. Wien, B. LXXVII. 1878.
(358) R. v. Willemoes-Suhm. "Biologische Beobachtungen ü. niedrige Meeresthiere." Zeit. f. wiss. Zool. Bd. XXI. 1871.
XIII. DISCOPHORA[143]
[143] The Discophora are divided into the following groups: I. Rhyncobdellidæ. II. Gnathobdellidæ. III. Branchiobdellidæ.
The eggs of the Discophora, each enclosed in a delicate membrane, are enveloped in a kind of mucous case formed by a secretion of the integument, which hardens into a capsule or cocoon. In each cocoon there are a limited number of eggs surrounded by albumen. The cocoons are attached to water plants, etc. In Clepsine the embryos leave the cocoon very soon after they get rid of the egg membrane, but in Nephelis they remain within the cocoon for a very much longer period (27-28 days after hatching). The young of Clepsine, after their liberation, attach themselves to the ventral surface of their parent.
Our knowledge of the development of the Discophora is in a very unsatisfactory state; but sufficient is known to shew that it has very many points in common with that of the Oligochæta, and that the Discophora are therefore closely related to the Chætopoda. In Clepsine there is an epibolic gastrula, and mesoblastic bands like those in Euaxes are also formed. In Nephelis however the segmentation is very abnormal, and the formation of the germinal layers cannot easily be reduced to an invaginate gastrula type, though probably it is modified from such a type. Mesoblastic bands similar to those in the Oligochæta occur in this form also.
The embryology of Clepsine, which will serve as type for the Leeches without jaws (Rhyncobdellidæ), has recently been studied by Whitman (No. 365), and that of Nephelis, which will serve as type for the Leeches with jaws (Gnathobdellidæ), has been studied by Bütschli (No. 359). The early history of both types is imperfectly known[144].
[144] Hoffmann's account (No. 36) is so different from that of other observers that I have been unable to make any use of it.
Formation of the layers.
Clepsine. It is necessary to give a full account of the segmentation of Clepsine, as the formation of the germinal layers would be otherwise unintelligible.
Segmentation commences with the division of the ovum into two unequal spheres by a vertical cleavage passing from the animal to the vegetative pole. By a second vertical cleavage the large segment is divided into two unequal parts, and the small one into two equal parts. Of the four segments so produced three are relatively small, and one, placed at the posterior end, is large. Each of the four segments next gives rise to a small cell at the animal pole. These small cells form the commencement of the epiblast, and, according to Whitman, the mouth is eventually placed in their centre. Such a position for the mouth, at the animal pole, is extremely unusual, and the statements on this head require further confirmation.
[FIG. 158. TWO VIEWS OF THE LARVA OF CLEPSINE. (After Whitman.)
o. oral extremity; m. mouth; pr. germinal streak. A. This figure shews the blastoderm (shaded) with a thickened edge formed by the primitive (i.e. mesoblastic) streaks with the four so-called neuroblasts posteriorly. The vitelline spheres are left without shading. B. represents an embryo in which the blastoderm has enclosed the yolk, and in which the division into segments has taken place. At the hind end are shewn the so-called neuroblasts forming the termination of the germinal streak.]
The posterior large segment now divides into two, one of which is dorsal, and the other and larger ventral. The former I shall call with Whitman the neuroblast, and the latter the mesoblast. The mesoblast very shortly divides again. During the formation of the neuroblast and mesoblast additional epiblastic small cells are added from the three spheres which give rise to three of the primitive epiblast cells, which may now be called the vitelline spheres.
The neuroblast next divides into ten cells, of which the two smaller are soon broken up into epiblastic cells, while the remaining eight arrange themselves in two groups of four each, one group on each side at the posterior border of the epiblastic cap. The two mesoblasts also take up a position on the right and left sides immediately ventral to the four neuroblasts of each side. The neuroblasts and mesoblasts now commence to proliferate at their anterior border, and produce on each side a thickened band of cells underneath the edge of the cap of epiblast cells. Each of these bands is formed of a superficial quadruple[145] row of neuroblasts budded off from the four primary neuroblasts, and a deeper row of mesoblasts. The compound streaks so formed may be called the germinal streaks.
[145] According to Robin it is more usual for there to be only a triple row of primary neuroblasts.
The general appearance of the embryo as seen from the dorsal surface, after the appearance of the two germinal streaks, may be gathered from fig. 158 A. The epiblastic cap in this figure is shaded. The epiblastic cap, accompanied by the germinal streaks, now rapidly extends and encloses the three vitelline spheres by a process equivalent to that of an ordinary epibolic gastrula; but the front and hind ends of the streaks remain practically stationary. Owing to this mode of growth the edges of the epiblastic cap and the germinal streaks meet in a linear fashion along the ventral surface of the embryo (fig. 159, A and B). The germinal streaks first meet anteriorly (B) and their junction is then gradually continued backwards. The process is completed at about the time of hatching.
During the above changes the nuclei of the vitelline spheres pass to the surface and rapidly divide. Eventually, together with part of the protoplasm of the vitelline spheres, they appear to give rise to a layer of hypoblastic cells. This layer encloses the remains of the vitelline spheres, which become the yolk.
[FIG. 159. TWO EMBRYOS OF CLEPSINE IN WHICH THE GERMINAL STREAKS HAVE PARTIALLY MET ALONG THE VENTRAL LINE. (After Robin.)
gs. germinal, i.e. mesoblastic streaks. The area covered by epiblast is shaded. The so-called neuroblasts at the end of the germinal streaks are shewn in B.]
At the front end of the germinal streaks, in a position corresponding with that of the four original epiblast cells, two depressions appear which coalesce to form the single oral invagination; in the centre of which are formed the mouth and pharynx by a second epiblastic invagination.
The most important point in connection with the above history is the fate of what have been called the germinal streaks. According to Whitman they are composed of two kinds of cells, viz. four rows of smaller superficial cells, which he calls neuroblasts, and, in the later stages at any rate, a row of deeper large cells, which he calls mesoblasts. As to the eventual fate of these cells he states that the neuroblasts uniting together in the median line form the rudiment of the ventral ganglionic chain, while the mesoblasts equally coalesce and give rise to the mesoblast. Such a mode of origin for a ventral ganglionic chain is, so far as I know, without a parallel in the whole animal kingdom; and whatever evidence Whitman may have that the cells in question really do give rise to the nervous system he has not thought fit to produce it in his paper. He figures a section with the eight neuroblastic cells in the middle ventral line, and in the next stage described the nervous system is divided up into ganglia! The first stage, in which the so-called nervous system has the form of a single row of eight cells, is quite unlike any rudiment of the nervous system such as is usually met with in the Chætopoda, and not a single stage between this and a ganglionated cord is described or figured. Whitman, whose views seem to have been influenced by a peculiar, and in my opinion erroneous, theory of Rauber's about the relation of the neural groove of Vertebrata to the blastopore, does not seem to be aware that his determination of the fate of his neuroblasts requires any special support.
He quotes the formation of these parts in Euaxes (vide preceding Chapter, p. 324) as similar to that in Clepsine. In this comparison it appears to me probable that he may be quite correct, but the result of the comparison would be to shew that the neuroblasts and mesoblasts composed together a mesoblastic band similar to that of the Oligochæta. Till more evidence is brought forward by Whitman or some other observer in support of the view that the so-called neuroblasts have any share in forming the nervous system, they must in my opinion be regarded as probably forming, in conjunction with the mesoblasts, two simple mesoblastic bands. Kowalevsky has moreover briefly stated that he has satisfied himself that the nervous system in Clepsine originates from the epiblast--a statement which certainly could not be brought into harmony with Whitman's account.
Nephelis. Nephelis will form my type of the Gnathobdellidæ. The segmentation of this form has not yet been thoroughly investigated, but Bütschli's (No. 359) observations are probably the most trustworthy.
The ovum first divides into two, and then into four segments of which two are slightly smaller than the others. Four small cells which form the commencement of the epiblast are now formed. Three of them are derived by budding from the two larger and one of the smaller of the four cells, and the fourth from a subsequent division of one of the larger cells[146]. The three cells which assisted in the formation of the epiblast cells again give rise each to a small cell; and the small cells so formed constitute a layer underneath the epiblast which is the commencement of the hypoblast, while the cells from which they originated form the vitelline spheres. Shortly after the formation of the hypoblast, the large sphere which has hitherto been quiescent divides into two, one of which then gives rise in succession to two small epiblastic elements.
[146] Doubts have been cast by Whitman on the above account of the origin of the four epiblast cells.
The two large spheres, resulting from the division of the originally quiescent sphere, next divide again on the opposite side of the embryo, and form a layer of epiblast there; so that there is now on one side of the embryo (the ventral according to Robin) a layer of epiblast formed of six cells, and on the opposite side a layer formed of four cells. The two layers meet at the front border of the embryo and between them are placed the three large vitelline spheres. The two patches of epiblast cells now rapidly increase, and gradually spread over the three large vitelline spheres. Except where they meet each other at the front edge they leave uncovered a large part of the margin of the vitelline spheres.
While these changes have been taking place on the exterior, the hypoblast cells have increased in number (additional cells being probably derived from the three large vitelline spheres) and fill up in a column-like fashion a space which is bounded behind by the three vitelline spheres, and in front by the epiblast of the anterior end of the embryo. At the sides of the hypoblast the mesoblast has become established, probably as two lateral bands. The origin of the cells forming it has not yet been determined. The hypoblast cells in the succeeding stage arrange themselves round a central archenteric cavity, and at the same time rapidly increase in size and become filled with a secondary deposit of food-yolk. Shortly afterwards a mouth and thick-walled oesophagus are formed, probably from an epiblastic invagination. The mesoblast now forms two curved lateral bands at the two sides of the body, equivalent to the mesoblastic bands of the Chætopoda. The three vitelline spheres, still largely uncovered by the epiblast, lie at the posterior end of the body. The embryo grows rapidly, especially anteriorly, and the three vitelline spheres become covered by a layer of flattened epiblast cells. Around the oesophagus a cavity traversed by muscular fibres is established. Elsewhere there is no trace of such a cavity. The cephalic region becomes ciliated, and the dorsal part of it, which represents a rudimentary præ-oral lobe, is especially prominent. The cilia of the oral region are continued into the lumen of the oesophagus, and at a later period are prolonged, as in Lumbricus, along the median line of the ventral surface.
The mesoblastic bands would seem from Bütschli's observations, which receive confirmation from Kleinenberg's researches on Lumbricus, to be prolonged dorsally to the oesophagus into the cephalic region. Posteriorly they abut on the large vitelline spheres, which were supposed by Kowalevsky to give origin to them, and to play the same part as the large mesoblasts in Lumbricus. It has already been shewn that the function of the large cells in Lumbricus has been exaggerated, and Bütschli denies to them in Nephelis any share in the production of the mesoblast. It seems in fact probable that they are homologous with the three vitelline spheres of Clepsine; and that their primitive function is to give origin to the hypoblast. They are visible for a long time at the hind end of the embryo, but eventually break up into smaller cells, the fate of which is unknown.
The embryo of Hirudo would appear from the researches of Robin to develop in nearly the same way as that of Nephelis. The anterior part is not however ciliated. The three large posterior cells disappear relatively early.
General history of the larva.
The larva of Clepsine, at the time when the mesoblastic bands have met along the ventral line, is represented in fig. 158 B. It is seen to be already segmented, the process having proceeded pari passu with the ventral coalescence of the mesoblastic bands. The segments are formed from before backwards as in Chætopoda. The dorsal surface is flat and short, and the ventral very convex. The embryo about this time leaves its capsule, and attaches itself to its parent. It rapidly elongates, and the dorsal surface, growing more rapidly than the ventral, becomes at last the more convex. Eventually thirty-three post-oral segments become formed; of which the eight last coalesce to form the posterior sucker.
The general development of the body of Nephelis and Hirudo is nearly the same as that of Clepsine. The embryo passes from a spherical to an oval, and then to a vermiform shape. For full details the reader is referred to Robin's memoir.
The presence of a well-marked protuberance above the oesophagus, which forms the rudiment of a præ-oral lobe, has already been mentioned as characteristic of the embryo of Nephelis; no such structure is found in Clepsine.
History of the germinal layers and development of organs.
The epiblast. The epiblast is formed of a single layer of cells and early develops a delicate cuticle which is clearly formed quite independently of the egg membrane. It becomes raised into a series of transverse rings which bear no relation to the true somites of the mesoblast.
The nervous system. The nervous system is probably derived from the epiblast, but its origin still requires further investigation. The ventral cord breaks up into a series of ganglia, which at first correspond exactly with the somites of the mesoblast. Of these, four or perhaps three eventually coalesce to form the sub-oesophageal ganglion, and seven or eight become united in the posterior sucker.
It would appear from Bütschli's statements that the supra-oesophageal ganglion arises, as in Oligochæta, independently of the ventral cord.
Mesoblast. It has already been indicated that the mesoblast probably takes its origin both in Nephelis and Clepsine from the two mesoblastic bands which unite in the median ventral line. The further history of these bands is only imperfectly known. They become segmented from before backwards. The somites formed by the segmentation gradually grow upwards and meet in the dorsal line. Septa are formed between the somites probably in the same way as in the Oligochæta.
In Clepsine the mesoblastic bands are stated by Kowalevsky to become split into somatic and splanchnic layers, between which are placed the so-called lateral sinuses. These sinuses form, according to Whitman, a single continuous tube investing the alimentary tract; a tube which differs therefore to a very small extent from the normal body cavity of the Chætopoda. The somatic layer of mesoblast no doubt gives rise to the circular and longitudinal muscular layers of the embryo. The former is stated to appear the earliest, while the latter, as in the Oligochæta, first takes its origin on the ventral side.
A delicate musculature, formed mainly of transverse but also of longitudinal fibres, would appear to be developed independently of the mesoblastic bands in Nephelis and Hirudo (Rathke, Leuckart, Robin, and Bütschli). It develops apparently from certain stellate cells which are found between the walls of the alimentary tract and the skin, and which probably correspond to the system of contractile fibres which pass from the body wall to the alimentary tract through the segmentation cavity in the larva of Chætopoda, various Vermes and Mollusca[147].
[147] According to Robin this system of muscles becomes gradually strengthened and converted into the permanent system. Rathke on the other hand states that it is provisional, and that it is replaced by the muscles developed from the mesoblastic somites. It is possible to suppose that it may really become incorporated in the latter system.
The mesoblast, so far as is known, gives rise, in addition to the parts already mentioned, to the excretory organs, generative organs, vascular system, etc.
Excretory organs. There are found in the embryo of Nephelis and Hirudo certain remarkable provisional excretory organs the origin and history of which is not yet fully made out. In Nephelis they appear as one (according to Robin, No. 364), or (according to Bütschli, No. 359) as two successive pairs of convoluted tubes on the dorsal side of the embryo, which are stated by the latter author to develop from the scattered mesoblast cells underneath the skin. At their fullest development they extend, according to Robin, from close to the head to near the ventral sucker. Each of them is U-shaped, with the open end forwards, each limb of the U being formed by two tubes united in front. No external opening has been clearly made out. Semper believed that the tubes were continuous with the three posterior vitelline cells, but this has been shewn not to be the case. Fürbringer[148] is inclined from his own researches to believe that they open laterally. They contain a clear fluid.
[148] Morphologisches Jahrbuch, Vol. IV. p. 676. He further speaks of the tube as "feinverzweigt u. netzförmig verästelt," but whether from his own observations is not clear.
In Hirudo, Leuckart (No. 362) has described three similar pairs of organs the structure of which he has fully elucidated. They are situated in the posterior part of the body, and each of them commences with an enlargement from which a convoluted tube is continued for some distance backwards; it then turns forwards again and afterwards bends upon itself to open to the exterior. The anterior part is broken up into a kind of labyrinthic network.
The true segmental organs are found in a certain number of the segments and are stated (Whitman) to develop from groups of mesoblast cells. Their origin requires however further investigation.
A double row of colossal cells on each side of the body has been described in Clepsine by Whitman as derived from the mesoblastic plates. These cells (fig. 58 B), which he calls segment-cells, lie opposite the walls of the septa. The inner row is stated to be connected with the segmental organs. Their eventual history is unknown, but they are conjectured by Whitman to be the mother cells of the testes.
The alimentary tract. This is formed primitively of two parts--the epiblastic stomodæum--forming mouth, pharynx, and oesophagus, and the hypoblastic mesenteron. The anus is formed very late as a simple perforation immediately dorsal to the posterior sucker.
In Clepsine, where there is an epibolic gastrula, the rudiment of the mesenteron is at first formed of the three vitelline spheres, from the surface of which a true hypoblastic layer enclosing a central yolk mass becomes differentiated, as already described. The mesenteric sack so formed is constricted by the growth of the mesoblastic septa into a series of lobes, while the posterior part forms a narrow and at first very short tube opening by the anus.
The lobed region forms the sacculated stomach of the adult. The sacculations of the stomach by their mode of origin necessarily correspond with the segments. In the adult however the anterior lobe is really double and has two divisions for the two segments it fills, while the posterior lobe, which, as is well known, extends backwards parallel with the rectum, is composed of five segmental sacculations. In connection with the stomodæum a protrusible pharynx is developed.
In Hirudo and Nephelis the mesenteron has from the first a sack-like form. The cells which compose the sack give rise to a secondary deposit of food-yolk. The further changes are practically the same as in Clepsine. In Hirudo the posterior sacculation of the stomach is primitively unpaired. The jaws are formed at about the same time as the eyes as protuberances on the wall of the oral cavity.
BIBLIOGRAPHY.
(359) O. Bütschli. "Entwicklungsgeschichtliche Beiträge (Nephelis)." Zeit. f. wiss. Zool. Vol. XXIX. 1877.
(360) E. Grube. Untersuchungen üb. d. Entwicklung d. Anneliden. Königsberg, 1844.
(361) C. K. Hoffmann. "Zur Entwicklungsgeschichte d. Clepsineen." Niederländ. Archiv f. Zool. Vol. IV. 1877.
(362) R. Leuckart. Die menschlichen Parasiten (Hirudo), Vol. I. p. 686, et seq.
(363) H. Rathke. Beit. z. Entwicklungsgesch. d. Hirudineen. Leipzig, 1862.
(364) Ch. Robin. Mém. sur le Développement embryogenique des Hirudinées. Paris, 1875.
(365) C. O. Whitman. "Embryology of Clepsine." Quart. J. of Micro. Science, Vol. XVIII. 1878.
[Vide also C. Semper (No. 355) and Kowalevsky (No. 342) for isolated observations.]
XIV. GEPHYREA[149]
[149] The following scheme shews the classification of the Gephyrea adopted in the present chapter: I. Gephyrea nuda. { (1) Inermia. { (2) Armata. II. Gephyrea tubicola. (Phoronis).
It is convenient for the purposes of embryology to divide the Gephyrea into two groups, viz. (1) Gephyrea nuda or true Gephyrea; and (2) Gephyrea tubicola formed by the genus Phoronis.
GEPHYREA NUDA.
Segmentation and formation of the layers.
An embolic or epibolic gastrula is characteristic of the Gephyrea, and the blastopore appears, in some cases at any rate (Phascolosoma, Thalassema), to become the mouth.
Bonellia. In Bonellia (Spengel, No. 370) the segmentation is unequal but complete, and, as in many Molluscs etc., the ovum exhibits before its commencement a distinction into a protoplasmic and a yoke pole. The ovum first divides into four equal segments, each of them formed of the same constituents as the original ovum. At the animal pole four small cells, entirely formed of protoplasm, are next formed by an equatorial furrow. They soon place themselves in the intervals between the large spheres. Four small cells are again budded off from the large spheres and the eight small cells then divide. By a further continuation of the division of the existing small cells, and the formation of fresh ones from the large spheres, a layer of small cells is eventually formed, which completely envelops the four large spheres except for a small blastopore at the vegetative pole of the ovum (fig. 160 A). The large spheres continue to give rise to smaller cells which however no longer take a superficial position but lie within the layer of small cells, and give rise to the hypoblast (fig. 160 B). The small cells become the epiblast, and at the blastopore they curl inwards (fig. 160 B) and give rise to a layer of cells, which appears to extend as an unbroken sheet between the epiblast and hypoblast, and to form the mesoblast. The blastopore now closes up, but its position in relation to the parts of the embryo has not been made out.
[FIG. 160. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.)
A. Stage when the four hypoblast cells are nearly enclosed. B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore. ep. epiblast; me. mesoblast; bl. blastopore.]
In Phascolosoma (Selenka, No. 369) the ovum, enclosed in a porous zona radiata, divides into two unequal spheres, of which the smaller next divides into two and then into four. An invagination takes place which is intermediate between the embolic and the epibolic types. The small cells, the number of which is increased by additions from the large sphere, divide, and grow round the large sphere. The latter in the meantime also divides, and the cells produced from it form on the one hand a small sack which opens by the blastopore, and on the other they fill up the segmentation cavity, and become the mesoblast and blood corpuscles. The blastopore becomes the permanent mouth.
Larval forms and development of organs.
Amongst the Gephyrea armata the larva has as a rule (Thalassema, Echiurus) the characters of a trochosphere, and closely approaches the typical form characteristic of the larva of Polygordius, often known as Lovén's larva. In Bonellia this larval form is less perfectly preserved.
Echiurus. In Echiurus (Salensky, No. 368) the youngest known larva has all the typical trochosphere characters (fig. 161). It is covered with cilia and divided into a præ-oral lobe and post-oral region of nearly equal dimensions. There is a double ciliated ring which separates the two sections of the body as in the larva of Polygordius: the mouth (m) opens between its two elements. The alimentary canal is divided into a stomodæum with a ventral opening, a large stomach, and a short intestine opening by a terminal anus (an). Connecting the oesophagus with the apex of the præ-oral lobe is the usual contractile band, and at the insertion of this band is a thickening of the epiblast which probably represents the rudiment of the supra-oesophageal ganglion. A ventral nerve cord is stated by Salensky to be present, but his observations on this point are not quite satisfactory.
[FIG. 161. LARVA OF ECHIURUS. (After Salensky.)
m. mouth; an. anus; sg. supra-oesophageal ganglion (?).]
The metamorphosis is accompanied by the loss of swimming power, and consists in the enlargement of the post-oral portion of the trunk, and in the simultaneous reduction of the præ-oral lobe, which remains however permanently as the cylindrical proboscis. A groove which terminates posteriorly at the mouth is very early formed on its ventral side. The ciliated rings gradually disappear during the metamorphosis.
Of the further external changes the most important are (1) the early appearance round the anal end of the body of a ring of bristles; and (2) the appearance of a pair of ventral setæ in the anterior part of the body. The anterior ring of bristles characteristic of the adult Echiurus does not appear till a late period.
Of the internal changes the earliest is the formation of the anal respiratory sacks. With the growth of the posterior part of the trunk the intestine elongates, and becomes coiled.
Bonellia. The embryo of Bonellia, while still within the egg, retains a spherical form and acquires an equatorial band of cilia, behind which a second narrower band is soon established, while in front of the first one a pair of eye-spots becomes formed (fig. 162 A). The embryo on becoming hatched rapidly elongates, while at the same time it becomes dorso-ventrally flattened and acquires a complete coating of cilia (fig. 162 B). According to Spengel it resembles at this time in its form and habits a rhabdocoelous Turbellarian. The anterior part is however somewhat swollen and presents an indication of a præ-oral lobe.
[FIG. 162. THREE STAGES IN THE DEVELOPMENT OF BONELLIA. (After Spengel.)
A. Larva with two ciliated bands and two eye-spots. B. Ripe larva from the dorsal surface. C. Young female Bonellia from the side. al. alimentary tract; m. mouth; se. provisional excretory tube; s. ventral hook; an.v. anal vesicle.]
During the above changes important advances are made in the formation of the organs from the embryonic layers.
The epiblast acquires a superficial cuticula, which is perhaps directly derived from the vitelline membrane. The nervous system is also formed, probably from the epiblast. The band-like supra-oesophageal ganglion is the first part of the nervous system formed, and appears to be undoubtedly derived from the epiblast. The ventral cord arises somewhat later, but the first stages in its development have not been satisfactorily traced. It is continuous with the supra-oesophageal band which completely girths the oesophagus without exhibiting any special dorsal enlargement. After the ventral cord has become completely separated from the epiblast a central fibrous mass becomes differentiated in it, while the lateral parts are composed of ganglion cells. In the arrangement of its cells it presents indications of being composed of two lateral halves. It is, however, without ganglionic swellings.
The mesoblast, though at first very thin, soon exhibits a differentiation into a splanchnic and somatic layer--though the two do not become distinctly separated by a body cavity. The somatic layer rapidly becomes thicker, and enlarges laterally to form two bands united dorsally and ventrally by narrow, thinner bands. The outermost parts of each of these bands become differentiated into an external circular and an internal longitudinal layer of muscles. In the præ-oral lobe the mesoblast assumes a somewhat vacuolated character.
The hypoblast cells form a complete layer round the four yolk cells from which they arise (fig. 162 B, al), but at first no alimentary lumen is developed. The oesophagus appears during this period as an, at first solid, but subsequently hollow, outgrowth of the hypoblast towards the epiblast.
The metamorphosis of the larva into the adult female Bonellia commences with the conversion of many of the indifferent mesoblast cells into blood corpuscles, and the introduction into the body cavity of a large amount of fluid, which separates the splanchnic and somatic layers of mesoblast. The fluid is believed by Spengel to be sea-water, introduced by two anal pouches, the development of which is described below.
The body cavity is lined by a peritoneum, and very soon distinct vessels, formed by folds of the peritoneum, become established. Of these there are three trunks, two lateral and a median in the præ-oral lobe (proboscis), and in the body a ventral trunk above the nerve cord, and an intestinal trunk opening anteriorly into the ventral one. The vessels appear to communicate with the body cavity.
In the course of the above changes the two ciliated bands disappear, the hinder one first. The cilia covering the general surface become atrophied, with the exception of those on the ventral side of the præ-oral lobe. The latter structure becomes more prominent; the stellate mesoblast cells, which fill up its interior, become contractile, and it gives rise to the proboscis (fig. 162 C).
At the point where the oesophageal protuberance joined the epiblast at a previous stage the mouth becomes established (fig. 162 C, m), and though it is formed subsequently to the atrophy of the anterior ciliated band, yet there is evidence that it is potentially situated behind this band. The lumen of the alimentary canal becomes established by the absorption of the remains of the four central cells. The anus is formed on the ventral side of the posterior end of the body, and close to it the pouches already noticed grow out from the hindermost part of the alimentary tract (fig. 162 C, an.v). They are at first simple blind pouches, but subsequently open into the body cavity[150]. They become the anal pouches of the adult. There is present when the mouth is first formed a peculiar process of the alimentary tract projecting into the præ-oral lobe, which appears to atrophy shortly afterwards.
[150] The fact that these pouches are outgrowths of the alimentary tract appears to preclude the possibility of their being homologous with excretory tubes of the Platyelminthes and Rotifera.
After the formation of the mouth, there are formed on the ventral side of and slightly behind it (1) anteriorly a pair of tubes, which appear to be provisional excretory organs and soon disappear (fig. 162 C, sc); and (2) behind them a pair of bristles (s) which remain in the adult. The formation of the permanent excretory (?) organ (oviduct and uterus) has not been followed out. The ovary appears very early as a differentiation of the epithelium lining the ventral vessel.
The larvæ, which become the minute parasitic males, undergo a very different and far less complete metamorphosis than those which become females. They attach themselves to the proboscis of an adult female, and lose their ciliated bands. Germinal cells make their appearance in the mesoblast, which form spherical masses, and, like the germinal balls in the female ovary, consist of a central cell, and an epithelium around it. The central cell becomes very large, while the peripheral cells give rise to the spermatozoa. A body cavity becomes developed in the larvæ, into which the spermatic balls are dehisced. Neither mouth nor anus is formed. The further changes have not been followed out.
The larval males make their way into the oesophagus of the female, where they no doubt live for some time, and probably become mature, though the seminal pouch of the adult is not found in many of the males living in the oesophagus. When mature the males leave the oesophagus, and pass into the uterus.
Phascolosoma. Cilia appear in Phascolosoma (Selenka, No. 369) while the ovum is still segmenting. After segmentation they form a definite band immediately behind the mouth, which divides the larva into two hemispheres--a præ-oral and a post-oral. A præ-oral band of cilia is soon formed close to the post-oral band, and at the apex of the præ-oral lobe a tuft of cilia also appears.
The larva has now the characters of a trochosphere, but differs from the typical trochosphere in the post-oral part of the ciliated equatorial ring being more important than the præ-oral, and in the absence of an anus.
The metamorphosis commences very early. The trunk rapidly elongates, and the præ-oral lobe becomes relatively less and less conspicuous. The zona radiata becomes the larval cuticle.
Three pairs of bristles are formed on the trunk, of which the posterior pair appears first, then the anterior, and finally the middle pair: an order of succession which clearly proves they can have no connection with a true segmentation.
The tentacles become developed between the two parts of the ciliated ring, and finally the præ-oral lobe, unlike what takes place in the Gephyrea armata, nearly completely vanishes.
The anus appears fairly late on the dorsal surface, and the ventral nerve cord is established as an unganglionated thickening of the ventral epiblast.
GEPHYREA TUBICOLA.
The larva of Phoronis was known as Actinotrocha long before its connection with Phoronis was established by Kowalevsky (No. 372). There is a complete segmentation leading to the formation of a blastosphere, which is followed by an invagination, the opening of which is said by Kowalevsky to remain as the mouth[151]. It is at first terminal, but on the development of a large præ-oral lobe it assumes a ventral position. The anus is formed at a later period at the posterior end of the body.
[151] Kowalevsky states that what I have called the mouth is the anus, but his subsequent descriptions shew that he has transposed the mouth and anus in the embryo, and that the opening, which he asserts to be the anus, is in reality the mouth.
[FIG. 163. A SERIES OF STAGES IN THE DEVELOPMENT OF PHORONIS FROM ACTINOTROCHA. (After Metschnikoff.)
A. Young larva. B. Larva after the formation of post-oral ring of tentacles. C. Larva with commencing invagination to form the body of Phoronis. D. Invagination partially everted. E. Invagination completely everted. m. mouth; an. anus; iv. invagination to form the body of Phoronis.]
The youngest free larva observed by Metschnikoff (No. 373) was less developed than the oldest larva found by Kowalevsky. It probably belongs to a different species. The body is uniformly ciliated (fig. 163 A). There is a large contractile præ-oral lobe, and the body ends behind in two processes. The mouth (m) is ventral, and the anus (an) dorsal, and not terminal as in Kowalevsky's larva.
The alimentary tract is divided into stomodæum, stomach and intestine. The two processes at the hind end of the body are the rudiments of the first-formed pair of the arms which are so characteristic of the fully developed Actinotrocha. A second pair of arms next become established on the dorsal side of the previously existing pair, and the region where the anus is placed grows out as a special process. New pairs of arms continue to be formed in succession dorsalwards and forwards, and soon constitute a complete oblique post-oral ring (fig. 163 B). They are covered by long cilia. Round the anal process a very conspicuous ciliated ring also becomes established.
At the period when five pairs of arms are present a delicate membrane becomes visible on the ventral side of the intestine which joins the somatic mesoblast anteriorly. This membrane is the rudiment of the future ventral vessel. The somatic mesoblast is present even before this period as a delicate layer of circular muscular fibres.
When six pairs of arms have become formed an involution (fig. 163 C, iv) appears on the ventral side, immediately behind the ring of arms. This involution consists both of the epiblast and somatic mesoblast. It grows inwards towards the intestine, and, increasing greatly in length, becomes at the same time much folded.
When it has reached its full development the critical period of the metamorphosis of Actinotrocha into Phoronis is reached, and is completed in about a quarter of an hour. The ventral involution becomes evoluted (fig. 163 D), just as one might turn out the finger of a glove which had been pulled inwards. When the involution has been to a certain extent everted, the alimentary canal passes into it, and at the same time the body of the larva becomes violently contracted. By the time the evagination is completed it forms (fig. 162 E) a long conical body, containing the greater part of the alimentary tract, and constituting the body of the young Phoronis. The original anal process remains on the dorsal side as a small papilla (fig. 162 E, an).
While these changes have been taking place the præ-oral lobe has become much contracted, and partly withdrawn into the stomodæum. At the same time the arms have become bent forward, so as to form a ring round the mouth. Their bases become much thickened. The metamorphosis is completed by the entire withdrawal of the præ-oral lobe within the oesophagus, and by the casting off of the ends of the arms, their bases remaining as the circumoral ring of tentacles, which form however a lophophore rather than a complete ring. The peri-anal ring of cilia is also thrown off, and the anal process withdrawn into the body of the young Phoronis. There are now three longitudinal vascular trunks, united anteriorly by a circular vessel which is prolonged into the tentacles.
General Considerations.
The development of Phoronis is so different from that of the other Gephyrea that further investigations are required to shew whether Phoronis is a true Gephyrean. Apart from its peculiar metamorphosis Actinotrocha is a very interesting larval form, in that it is without a præ-oral ciliated ring, and that the tentacles of the adult are derived from a true post-oral ring, prolonged into arm-like processes.
The other Gephyrea present in their development an obvious similarity to the normal Chætopoda, but their development stops short of that of the Chætopoda, in that they are clearly without any indications of a true segmentation. In the face of what is known of their development it is hardly credible that they can represent a degenerate Chætopod phylum in which segmentation has become lost. Further than this the Gephyrea armata seem in one respect to be a very primitive type in that they retain through life a well-developed præ-oral lobe, which constitutes their proboscis. In almost all other forms, except Balanoglossus, the larval præ-oral lobe becomes reduced to a relatively insignificant anterior part of the head.
BIBLIOGRAPHY.
Gephyrea nuda.
(366) A. Kowalevsky. Sitz. d. zool. Abth. d. III. Versam. russ. Naturj. (Thalassema). Zeit. f. wiss. Zool. Vol. XXII. 1872, p. 284. (367) A. Krohn. "Ueb. d. Larve d. Sipunculus nudus nebst Bemerkungen," etc. Müller's Archiv, 1857.
(368) M. Salensky. "Ueber die Metamorphose d. Echiurus." Morphologisches Jahrbuch, Bd. II.
(369) E. Selenka. "Eifurchung u. Larvenbildung von Phascolosoma elongatum." Zeit. f. wiss. Zool. 1875, Bd. XXV. p. 1.
(370) J. W. Spengel. "Beiträge z. Kenntniss d. Gephyreen (Bonellia)." Mittheil. a. d. zool. Station z. Neapel, Vol. I. 1879.
Gephyrea tubicola (Actinotrocha).
(371) A. Krohn. "Ueb. Pilidium u. Actinotrocha." Müller's Archiv, 1858.
(372) A. Kowalevsky. "On anatomy and development of Phoronis," Pétersbourg, 1867. 2 Pl. Russian. Vide Leuckart's Bericht, 1866-7.
(373) E. Metschnikoff. "Ueber d. Metamorphose einiger Seethiere (Actinotrocha)." Zeit. f. wiss. Zool. Bd. XXI. 1871.
(374) J. Müller. "Bericht üb. ein. Thierformen d. Nordsee." Müller's Archiv, 1846.
(375) An. Schneider. "Ueb. d. Metamorphose d. Actinotrocha branchiata." Müller's Arch. 1862.
XV. CHÆTOGNATHA, MYZOSTOMEA AND GASTROTRICHA
The present chapter deals with three small isolated groups, which only resemble each other in that the systematic position of all of them is equally obscure.
Chætognatha.
The discoveries of Kowalevsky (No. 378) confirmed by Bütschli (No. 376) with reference to the development of Sagitta, though they have not brought us nearer to a knowledge of the systematic position of this remarkable form, are nevertheless of great value for the more general problems of embryology. The development commences after the eggs are laid. The segmentation is uniform, and a blastosphere, formed of a single layer of columnar cells, is the product of it. An invagination takes place, the opening of which narrows to a blastopore situated at the pole of the embryo opposite that at which the mouth subsequently appears (fig. 164 A). The simple archenteron soon becomes anteriorly divided into three lobes, which communicate freely with the still single cavity behind (fig. 164 B). The two lateral lobes are destined to form the body cavity, and the median lobe the alimentary tract of the adult. An invagination soon arises at the opposite pole of the embryo to the blastopore and forms the mouth and oesophagus (fig. 164 B and C, m).
[FIG. 164. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after Bütschli and B after Kowalevsky.) The three embryos are represented in the same positions.
A. The gastrula stage. B. A succeeding stage in which the primitive archenteron is commencing to be divided into three parts, the two lateral of which are destined to form the body cavity. C. A later stage in which the mouth involution (m) has become continuous with the alimentary tract, and the blastopore has become closed. m. mouth; al. alimentary canal; ae. archenteron; bl.p. blastopore; pv. perivisceral cavity; sp. splanchnopleuric mesoblast; so. somatopleuric mesoblast; ge. generative organs.]
At the gastrula stage there is formed a paired mass destined to give rise to the generative organs. It arises as a prominence of six cells, projecting from the hypoblast at the anterior pole of the archenteron, and soon separates itself as a mass, or probably a pair of masses, lying freely in the cavity of the archenteron (fig. 164 A, ge). When the folding of the primitive cavity takes place the generative rudiment is situated at the hind end of the median lobe of the archenteron in the position represented in fig. 164 C, ge.
An elongation of the posterior end of the embryo now takes place, and the embryo becomes coiled up in the egg, and when eventually hatched sufficiently resembles the adult to be recognisable as a young Sagitta.
Before hatching takes place various important changes become manifest. The blastopore disappears after being carried to the ventral surface. The middle section of the trilobed region of the archenteron becomes separated from the unpaired posterior part, and forms a tube, blind behind, but opening in front by the mouth (fig. 165 A, al). It constitutes the permanent alimentary tract, and is formed of a pharyngeal epiblastic invagination, and a posterior hypoblastic section derived from the primitive archenteron. The anus is apparently not formed till comparatively late. After the isolation of the alimentary tract the remainder of the archenteron is formed of two cavities in front, which open freely into a single cavity behind (fig. 165 A). The whole of it constitutes the body cavity and its walls the mesoblast. The anterior paired part becomes partitioned off into a head section and a trunk section (fig. 165 A and B). The former constitutes a pair of distinct cavities (c.pv) in the head, and the latter two cavities opening freely into the unpaired portion behind. At the junction of the paired cavities with the unpaired cavity are situated the generative organs (ge). The inner wall of each of the paired cavities forms the splanchnopleuric mesoblast, and the outer wall of the whole the somatic mesoblast. The inner walls of the posterior cavities unite above and below the alimentary tract, and form the dorsal and ventral mesenteries, which divide the body cavity into two compartments in the adult. Before the hatching of the embryo takes place this mesentery is continued backwards so as to divide the primitively unpaired caudal part of the body cavity in the same way.
[FIG. 165. TWO VIEWS OF A LATE EMBRYO OF SAGITTA. A. from the dorsal surface. B. from the side. (After Bütschli.)
m. mouth; al. alimentary canal; v.g. ventral ganglion (thickening of epiblast); ep. epiblast; c.pv. cephalic section of body cavity; so. somatopleure; sp. splanchnopleure; ge. generative organs.]
From the somatic mesoblast of the trunk is derived the single layer of longitudinal muscles of Sagitta, and part of the epithelioid lining of the body cavity. The anterior termination of the trunk division of the body cavity is marked in the adult by the mesentery dividing into two laminæ, which bend outwards to join the body wall.
The cephalic section of the body cavity seems to atrophy, and its walls to become converted into the complicated system of muscles present in the head of the adult Sagitta.
In the presence of a section of the body cavity in the head the embryo of Sagitta resembles Lumbricus, Spiders, etc.
The generative rudiment of each side divides into an anterior and a posterior part (fig. 165, ge). The former constitutes the ovary, and is situated in front of the septum dividing the tail from the body; and the latter, in the caudal region of the trunk, forms the testis.
The nervous system originates from the epiblast. There is a ventral thickening (fig. 165 B, v.g) in the anterior region of the trunk, and a dorsal one in the head. The two are at first continuous, and on becoming separated from the epiblast remain united by thin cords.
The ventral ganglion is far more prominent during embryonic life than in the adult. Its position and early prominence in the embryo perhaps indicate that it is the homologue of the ventral cord of Chætopoda[152].
[152] Langerhans has recently made some important investigations on the nervous system of Sagitta, and identifies the ventral ganglion with the parieto-splanchnio ganglia of Molluscs, while he has found a pair of new ganglia, the development of which is unknown, which he calls the suboesophageal or pedal ganglia. The embryological facts do not appear to be in favour of these interpretations.
BIBLIOGRAPHY.
(376) O. Bütschli. "Zur Entwicklungsgeschichte der Sagitta." Zeitschrift f. wiss. Zool., Vol. XXIII. 1873.
(377) C. Gegenbaur. "Über die Entwicklung der Sagitta." Abhand. d. naturforschenden Gesellschaft in Halle, 1857.
(378) A. Kowalevsky. "Embryologische Studien an Würmern u. Arthropoden." Mém. Acad. Pétersbourg, VII. sér., Tom. XVI., No. 12. 1871.
MYZOSTOMEA.
The development of these peculiar parasites on Crinoids has been investigated by Metschnikoff (No. 380), Semper (No. 381), and Graff (No. 379).
The segmentation is unequal, and would appear to be followed by an epibolic invagination. The outer layer of cells (epiblast) becomes covered with cilia, and the inner is transformed into a non-cellular (?) central yolk mass. At this stage the larva is hatched, and commences to lead a free existence. In the next stage observed by Metschnikoff, the mouth, oesophagus, stomach, and anus had become developed; and two pairs of feet were present. In both of these feet Chætopod-like setæ were present, which in the hinder pair were simple fine bristles without a terminal hook. The papilliform portion of the foot is at first undeveloped. The feet become successively added, like Chætopod segments, and the stomach does not become dendriform till the whole complement of feet (5 pairs) are present.
In the primitive covering of cilia, combined with a subsequent indication of segments in the formation of the feet and setæ, the larva of the Myzostomea shews an approximation to the Chætopoda, and the group is probably to be regarded as an early Chætopod type specially modified in connection with its parasitic habits.
BIBLIOGRAPHY.
(379) L. Graff. Das Genus Myzostoma. Leipzig, 1877.
(380) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Myzostomum." Zeit. f. wiss. Zool., Vol. XVI. 1866.
(381) C. Semper. "Z. Anat. u. Entwick. d. Gat. Myzostomum." Zeit. f. wiss. Zool., Vol. IX. 1858.
GASTROTRICHA.
A few observations of Ludwig on the winter eggs of Ichthydium larus shew that the segmentation is a total and apparently a regular one. It leads to the formation of a solid morula. The embryo has a ventral curvature, and the caudal forks are early formed as cuticular structures. By the time the embryo leaves the egg, it has almost reached the adult state. The ventral cilia arise some little time prior to the hatching.
BIBLIOGRAPHY.
(382) H. Ludwig. "Ueber die Ordnung Gastrotricha Metschn." Zeit. f. wiss. Zool., Vol. XXVI. 1876.
XVI. NEMATELMINTHES AND ACANTHOCEPHALA
NEMATELMINTHES[153].
[153] The following classification of the Nematoda is employed in this chapter: { Ascaridæ. { Strongylidæ. I. Nematoidea. { Trichinidæ. { Filaridæ { Mermithidæ. { Anguillulidæ. II. Gordioidea. III. Chætosomoidea.
Nematoidea. Although the ova of various Nematodes have formed some of the earliest, as well as the most frequent objects of embryological observation, their development is still but very imperfectly known. Both viviparous and oviparous forms are common, and in the case of the oviparous forms the eggs are usually enveloped in a hard shell. The segmentation is total and nearly regular, though the two first segments are often unequal. The relation of the segmentation spheres to the germinal layers is however only satisfactorily established (through the researches of Bütschli (No. 383)) in the case of Cucullanus elegans, a form parasitic in the Perch[154].
[154] The ova of Anguillula aceti are stated by Hallez to undergo a similar development to those of Cucullanus.
The early development of this embryo takes place within the body of the parent, and the egg is enveloped in a delicate membrane. After the completion of the early stages of segmentation the embryo acquires the form of a thin flat plate composed of two layers of cells (fig. 166 A and B). The two layers of this plate give rise respectively to the epiblast and hypoblast, and at a certain stage the hypoblastic layer ceases to grow, while the growth of the epiblastic layer continues. As a consequence of this the sides of the plate begin to fold over towards the side of the hypoblast (fig. 166 D.) This folding results in the formation of a remarkably constituted gastrula, which has the form of a hollow two-layered cylinder with an incompletely closed slit on one side (fig. 166 E, bl.p). This slit has the value of a blastopore. It becomes closed by the coalescence of the two edges, a process which commences posteriorly, and then gradually extends forwards. In front the blastopore never becomes completely closed, but remains as the permanent mouth. The embryo after these changes has a worm-like form, which becomes the more obvious as it grows in length and becomes curved (fig. 166 F).
[FIG. 166. VARIOUS STAGES IN THE DEVELOPMENT OF CUCULLANUS ELEGANS. (From Bütschli.)
A. Surface view of flattened embryo at an early stage in the segmentation. B. Side view of an embryo at a somewhat later stage, in optical section. C. Flattened embryo at the completion of segmentation. D. Embryo at the commencement of the gastrula stage. E. Embryo when the blastopore is reduced to a mere slit. F. Vermiform embryo after the division of the alimentary tract into oesophageal and glandular divisions. m. mouth; ep. epiblast; hy. hypoblast; me. mesoblast; oe. oesophagus; bl.p. blastopore.]
The hypoblast of the embryo gives rise to the alimentary canal, and soon becomes divided into an oesophageal section (fig. 166 F, oe) formed of granular cells, and a posterior division formed of clear cells. The mesoblast (fig. 166, me) takes its origin from certain special hypoblast cells around the mouth, and thence grows backwards towards the posterior end of the body.
The young Cucullanus becomes hatched while still in the generative ducts of its parent, and is distinguished by the presence of a remarkable thread-like tail. On the dorsal surface is a provisional boring apparatus in the form of a conical papilla. A firm cuticle enveloping the body is already present. In this condition it leaves its parent and host, and leads for a time a free existence in the water. Its metamorphosis is dealt with in another section.
The ova of the Oxyuridæ parasitic in Insects are stated by Galeb (No. 386) to take the form of a blastosphere at the close of segmentation. An inner layer is then formed by delamination. What the inner layer gives rise to is not clear, since the whole alimentary canal is stated to be derived from two buds, which arise at opposite ends of the body, and grow inwards till they meet.
The generative organs. The study of the development of the generative organs of Nematodes has led to some interesting results. In the case of both sexes the generative organs originate (Schneider, No. 390) from a single cell. This cell elongates and its nuclei multiply. After assuming a somewhat columnar form, it divides into (1) a superficial investing layer, and (2) an axial portion.
In the female the superficial layer is only developed distinctly in the median part of the column. In the course of the further development the two ends of the column become the blind ends of the ovary, and the axial tissue they contain forms the germinal tissue of nucleated protoplasm. The superficial layer gives rise to the epithelium of the uterus and oviduct. The germinal tissue, which is originally continuous, is interrupted in the middle part (where the superficial layer gives rise to the uterus and oviduct), and is confined to the two blind extremities of the tube.
In the male the superficial layer, which gives rise to the epithelium of the vas deferens, is only formed at the hinder end of the original column. In other respects the development takes place as in the female.
Gordioidea. The ovum of Gordius undergoes a regular segmentation. According to Villot (No. 391) it forms at the close of segmentation a morula, which becomes two-layered by delamination. The embryo is at first spherical, but soon becomes elongated.
By an invagination at the anterior extremity the head is formed. It consists of a basal portion, armed with three rings of stylets, and a conical proboscis, armed with three large stylets. When the larva becomes free the head becomes everted, though it remains retractile. By the time the embryo is hatched a complete alimentary tract is formed with an oral opening at the end of the proboscis, and a subterminal ventral anal opening. It is divided into an oesophagus and stomach, and a large gland opens into it at the base of the proboscis.
The body has a number of transverse folds, which give it a ringed appearance.
Metamorphosis and life history.
Nematoidea. Although a large number of Nematodes have a free existence and simple life history, yet the greater number of known genera are parasitic, and undergo a more or less complicated metamorphosis[155]. According to this metamorphosis they may be divided into two groups (which by no means closely correspond with the natural divisions), viz. those which have a single host, and those with two hosts. Each of these main divisions may be subdivided again into two.
[155] The following facts are mainly derived from Leuckart's exhaustive treatise (No. 388).
In the first group with one host the simplest cases are those in which the adult sexual form of parasite lays its eggs in the alimentary tract of its host, and the eggs are thence transported to the exterior. The embryo still in the egg, if favoured by sufficient warmth and moisture, completes its development up to a certain point, and, if then swallowed by an individual of the species in which it is parasitic in the adult condition, it is denuded of its shell by the action of the gastric juice, and develops directly into the sexual form.
Leuckart has experimentally established this metamorphosis in the case of Trichocephalus affinis, Oxyurus ambigua, and Heterakis vermicularis. The Oxyuridæ of Blatta and Hydrophilus have a similar life history (Galeb, No. 386), and it is almost certain that the metamorphosis of the human parasites, Ascaris lumbricoides and Oxyurus vermicularis, is of this nature.
A slightly more complicated metamorphosis is common in the genera Ascaris and Strongylus. In these cases the egg-shell is thin, and the embryo becomes free externally, and enjoys for a shorter or longer period a free existence in water or moist earth. During this period it grows in size, and though not sexual usually closely resembles the adult form of the permanently free genus Rhabditis. In some cases the free larva becomes parasitic in a freshwater Mollusc, but without thereby undergoing any change. It eventually enters the alimentary tract of its proper host and there become sexual.
As examples of this form of development worked out by Leuckart may be mentioned Dochmius trigonocephalus, parasitic in the dog, and Ascaris acuminata, in the frog. The human parasite Dochmius duodenale undergoes the same metamorphosis as Dochmius trigonocephalus.
A remarkable modification of this type of metamorphosis is found in Ascaris (Rhabdonema) nigrovenosa, which in its most developed condition is parasitic in the lungs of the frog (Metschnikoff, Leuckart, No. 388). The embryos pass through their first developmental phases in the body of the parent. They have the typical Rhabditis form, and make their way after birth into the frog's rectum. From this they pass to the exterior, and then living either in moist earth, or the fæces of the frog, develop into a sexual form, but are very much smaller than in the adult condition. The sexes are distinct, and the males are distinguished from the females by their smaller size, shorter and rounded tails, and thinner bodies. The females have paired ovaries with a very small number of eggs, but the testis of the males is unpaired. Impregnation takes place in the usual way, and in summer time about four embryos are developed in each female, which soon burst their egg-capsules, and then move freely in the uterus. Their active movements soon burst the uterine walls, and they then come to lie freely in the body cavity. The remaining viscera of the mother are next reduced to a finely granular material, which serves for the nutrition of the young forms which continue to live in the maternal skin. The larvæ eventually become free, and though in many respects different from the parent form which gave rise to them, have nevertheless the Rhabditis form. They live in water or slime, and sometimes become parasitic in water-snails; in neither case however do they undergo important changes unless eventually swallowed by a frog. They then pass down the trachea into the lungs and there rapidly develop into the adult form. No separate males have been found in the lungs of the frog, but it has been shewn by Schneider (No. 390) that the so-called females are really hermaphrodites; the same gland giving origin to both spermatozoa and ova, the former being developed before the latter[156]. The remarkable feature of the above life history is the fact that in the stage corresponding with the free larval stage of the previous forms the larvæ of this species become sexual, and give rise to a second free larval generation, which develops into the adult form on again becoming parasitic in the original host. It constitutes a somewhat exceptional case of heterogamy as defined in the introduction.
[156] Leuckart does not appear to be satisfied as to the hermaphroditism of these forms; and holds that it is quite possible that the ova may develop parthenogenetically.
Amongst the Nematodes with but a single host a remarkable parasite in wheat has its place. This form, known as Anguillula scandens, inhabits in the adult condition the ears of wheat, in which it lays its eggs. After hatching, the larvæ become encysted, but become free on the death of the plant. They now inhabit moist earth, but eventually make their way into the ears of the young wheat and become sexually mature.
The second group of parasitic Nematodes with two hosts may be divided into two groups, according to whether the larva has a free existence before passing into its first or intermediate host, or is taken into it while still in the egg. In the majority of cases the larval forms live in special connective-tissue capsules, or sometimes free in the tissues of their intermediate hosts; but the adults, as in the cases of other parasitic Nematodes, inhabit the alimentary tract.
The life history of Spiroptera obtusa may be cited as an example of a Nematode with two hosts in which the embryo is transported into its intermediate host while still within the egg. The adult of this form is parasitic in the mouse, and the ova pass out of the alimentary tract with the excreta, and may commonly be found in barns, etc. If one of the ova is now eaten by the meal-worm (larva of Tenebrio), it passes into the body cavity of this worm and undergoes further development. After about five weeks it becomes encapsuled between the 'fat bodies' of the meal-worm. It then undergoes an ecdysis, and, if the meal-worm with its parasites is now eaten by the mouse, the parasites leave their capsule and develop into the sexual form.
As examples of life histories in which a free state intervenes before the intermediate host, Cucullanus elegans and Dracunculus may be selected. The adult Cucullanus elegans is parasitic in the alimentary tract of the Perch and other freshwater fishes. It is a viviparous form, and the young after birth pass out into the water. They next become parasitic in Cyclops, passing in through the mouth, so into the alimentary tract, and thence into the body cavity. They soon undergo an ecdysis, in the course of which the oesophagus becomes divided into a muscular pharynx and true glandular oesophagus. They then grow rapidly in length, and at a second ecdysis acquire a peculiar beaker-like mouth cavity approaching that of the adult. They do not become encapsuled. No further development of the worm takes place so long as it remains in the Cyclops, but, if the Cyclops is now swallowed by a Perch, the worm undergoes a further ecdysis, and rapidly attains to sexual maturity.
The observations of Fedschenko on Dracunculus medinensis[157], which is parasitic in the subcutaneous connective tissue in Man, would seem to shew that it undergoes a metamorphosis very similar to that of Cucullanus. There is moreover a striking resemblance between the larvæ of the two forms. The larvæ of Dracunculus become transported into water, and then make their way into the body cavity of a Cyclops by boring through the soft skin between the segments on the ventral surface of the body. In the body cavity the larvæ undergo an ecdysis and further development. But on reaching a certain stage of development, though they remain a long time in the Cyclops, they grow no further. The remaining history is unknown, but probably the next host is man, in which the larva comes to maturity. In the adult condition only females of Dracunculus are known, and it has been suggested by various writers that the apparent females are in reality hermaphrodites, like Ascaris nigrovenosa, in which the male organs come to maturity before the female.
[157] Vide Leuckart, D. men. Par., Vol. II. p. 704.
Another very remarkable human parasite belonging to the same group as Dracunculus is the form known as Filaria sanguinis hominis, or Filaria Bancrofti[158].
[158] Vide D. P. Manson, "On the development of Filaria sanguinis hominis." Journal of the Linnean Society, Vol. XIV. No. 75.
The sexual form is parasitic in warm climates in the human tissues, and produces multitudes of larvæ which pass into the blood, and are sometimes voided with the urine. The larvæ in the blood do not undergo a further development, and unless transported to an intermediate host die before very long. Some, though as yet hardly sufficient, evidence has been brought forward to shew that if the blood of an infected patient is sucked by a mosquito the larvæ develop further in the alimentary tract of the mosquito, pass through a more or less quiescent stage, and eventually grow considerably in size, and on the death of the mosquito pass into the water. From the water they are probably transported directly or indirectly into the human intestines, and then bore their way into the tissues in which they are parasitic, and become sexually mature.
The well-known Trichina spiralis has a life history unlike that of other known Nematodes, though there can be little doubt that this form should be classified in respect to its life history with the last-described forms. The peculiarity of the life history of Trichina is that the embryos set free in the alimentary canal pass through the walls into the muscular tissues and there encyst; but do not in a general way pass out from the alimentary canal of one host and thence into a fresh host to encyst. It occasionally however happens that this migration does take place, and the life history of Trichina spiralis then becomes almost identical with that of some of the forms of the third type. Trichina is parasitic in man, and in swine, and also in the rat, mouse, cat, fox and other forms which feed upon them. Artificially it can be introduced into various herbivorous forms (rabbit, guinea-pig, horse) and even birds.
The sexual form inhabits the alimentary canal. The female is viviparous, and produces myriads of embryos, which pass into the alimentary canal of their host, through the walls of which they make their way, and travelling along lines of connective tissue pass into the muscles. Here the embryos, which are born in a very imperfect condition, rapidly develop, and eventually assume a quiescent condition in a space inclosed by sarcolemma. Within the sarcolemma a firm capsule is developed for each larva, which after some months becomes calcified; and after the atrophy of the sarcolemma a connective-tissue layer is formed around it. Within its capsule the larva can live for many years, even ten or more, without undergoing further development, but if at last the infected flesh is eaten by a suitable form, e.g. the infected flesh of the pig by man, the quiescent state of the larva is brought to a close, and sexual maturity is attained in the alimentary tract of the new host.
Gordioidea. The free larva of Gordius already described usually penetrates into the larva of Chironomus where it becomes encysted. On the Chironomus being eaten by some fish (Villot, No. 39) (Phoxinus lævis or Cobitis barbatula), it penetrates into the wall of the intestine of its second host, becomes again encysted and remains quiescent for some time. Eventually in the spring it leaves its capsule, and enters the intestine, and passes to the exterior with the fæces. It then undergoes a gradual metamorphosis, in the course of which it loses its ringed structure and cephalic armature, grows in length, acquires its ventral cord, and on the development of the generative organs loses the greater part of its alimentary tract.
Young examples of Gordius have often been found in various terrestrial carnivorous Insecta, but the meaning of this fact is not yet clear.
BIBLIOGRAPHY.
(383) O. Bütschli. "Entwicklungsgeschichte d. Cucullanus elegans." Zeit. f. wiss. Zool., B. XXVI. 1876.
(384) T. S. Cobbold. Entozoa. Groombridge and Son, 1864.
(385) T. S. Cobbold. Parasites: A Treatise on the Entozoa of Man and Animals. Churchill, 1879.
(386) O. Galeb. "Organisation et développement des Oxyuridés," &c. Archives de Zool. expér. et génér., Vol. VII. 1878.
(387) R. Leuckart. Untersuchungen üb. Trichina spiralis. 2nd ed. Leipzig, 1866.
(388) R. Leuckart. Die menschlichen Parasiten, Bd. II. 1876. (389) H. A. Pagenstecher. Die Trichinen nach Versuchen dargestellt. Leipzig, 1865.
(390) A. Schneider. Monographie d. Nematoden. Berlin, 1866.
(391) A. Villot. "Monographie des Dragoneaux" (Gordioidea). Archives de Zool. expér. et génér., Vol. III. 1874.
ACANTHOCEPHALA.
The Acanthocephala appear to be always viviparous. At the time of impregnation the ovum is a naked cell, and undergoes in this condition the earlier phases of segmentation.
The segmentation is unequal (Leuckart, No. 393), but whether there is an epibolic gastrula has not clearly been made out.
Before segmentation is completed there are formed round the ovum thick protecting membranes, which are usually three in number, the middle one being the strongest. After segmentation the central cells of the ovum fuse together to give rise to a granular mass, while the peripheral cells at a slightly later period form a more transparent syncytium. At the anterior end of the embryo there appears a superficial cuticle bearing in front a ring of hooks.
The embryo is now carried out with the excreta from the intestine of the vertebrate host in which its parent lives. It is then swallowed by some invertebrate host[159].
[159] Echin. proteus, which is parasitic in the adult state in many freshwater fish, passes through its larval condition in the body cavity of Gammarus pulex. Ech. angustatus, parasitic in the Perch, is found in the larval condition in the body cavity of Asellus aquatious. Ech. gigas, parasitic in swine, is stated by Schneider (No. 394) to pass through its larval stages in maggots.
In the intestine of the invertebrate host the larva is freed from its membranes, and is found to have a somewhat elongated conical form, terminating anteriorly in an obliquely placed disc, turned slightly towards the ventral surface and armed with hooks. Between this disc and the granular mass, already described as formed from the central cells of the embryo, is a rather conspicuous solid body. Leuckart supposes that this body may represent a rudimentary functionless pharynx, while the granular mass in his opinion is an equally rudimentary and functionless intestine. The body wall is formed of a semifluid internal layer surrounding the rudimentary intestine, if such it be, and of a firmer outer wall immediately within the cuticle.
The adult Echinorhyncus is formed by a remarkable process of development within the body of the larva, and the skin is the only part of the larva which is carried over to the adult.
In Echinorhyncus proteus the larva remains mobile during the formation of the adult, but in other forms the metamorphosis takes place during a quiescent condition of the larva.
The organs of the adult are differentiated from a mass of cells which appears to be a product of the central embryonic granular mass, and is called by Leuckart the embryonic nucleus. The embryonic nucleus becomes divided into four linearly arranged groups of cells, of which the hindermost but one is the largest, and very early differentiates itself into (1) a peripheral layer, and (2) a central mass formed of two distinct bodies. The peripheral layer of this segment grows forwards and backwards, and embraces the other segments, with the exception of the front end of the first one which is left uncovered. The envelope so formed gives rise to the splanchnic and somatic mesoblast of the adult worm. Of the four groups of cells within it the anterior gives rise to the proboscis, the next to the nerve ganglion, the third, formed of two bodies, to the paired generatives, and the fourth to the generative ducts. The whole of the above complex rapidly elongates, and as it does so the enveloping membrane becomes split into two layers; of which the outer forms the muscular wall of the body (somatic mesoblast), and the inner the muscular sheath of the proboscis and the so-called generative ligament enveloping the generative organs. The inner layer may be called the splanchnic mesoblast in spite of the absence of an intestine. The cavity between the two mesoblastic layers forms the body cavity.
The various parts of the adult continue to differentiate themselves as the whole increases in size. The generative masses very early shew traces of becoming differentiated into testes or ovaries. In the male the two generative masses remain spherical, but in the female become elongated: the rudiment of the generative ducts becomes divided into three sections in both sexes. The most remarkable changes are, however, those undergone by the rudiment of the proboscis.
In its interior there is formed a cavity, but the wall bounding the front end of the cavity soon disappears. By the time that this has taken place the body of the adult completely fills up the larval skin, to which it very soon attaches itself. The hollow rudiment of the proboscis then becomes everted, and forms a papilla at the end of the body, immediately adjoining the larval skin. This papilla, with the larval skin covering it, constitutes the permanent proboscis. The original larval cuticle is either now or at an earlier period thrown off and a fresh cuticle developed. The hooks of the proboscis are formed from cells of the above papilla, which grow through the larval skin as conical prominences, on the apex of which a chitinous hook is modelled. The remainder of the larval skin forms the skin of the adult, and at a later period develops in its deeper layer the peculiar plexus of vessels so characteristic of the Acanthocephala. The anterior oval appendages of the adult cutis, known as the lemnisci, are outgrowths from the larval skin.
The Echinorhyncus has with the completion of these changes practically acquired its adult structure; but in the female the ovaries undergo at this period remarkable changes, in that they break up into a number of spherical masses, which lie in the lumen of the generative ligaments, and also make their way into the body cavity.
The young Echinorhyncus requires to be transported to its permanent host, which feeds on its larval host, before attaining to sexual maturity.
BIBLIOGRAPHY.
(392) R. Greeff. "Untersuchungen ü. d. Bau u. Entwicklung des Echin. miliarius." Archiv f. Naturgesch. 1864.
(393) R. Leuckart. Die menschlichen Parasiten. Vol. II. p. 801 et seq. 1876.
(394) An. Schneider. "Ueb. d. Bau d. Acanthocephalen." Archiv f. Anat. u. Phys. 1868.
(395) G. R. Wagener. Beiträge z. Entwicklungsgeschichte d. Eingeweidewürmer. Haarlem, 1865.