The elementary nervous system

Front matter

MONOGRAPHS ON EXPERIMENTAL BIOLOGY

EDITED BY JACQUES LOEB, Rockefeller Institute T. H. MORGAN, Columbia University W. J. V. OSTERHOUT, Harvard University

BY G. H. PARKER, Sc.D.

Section I.—effector Systems

+ SECTION II.—RECEPTOR-EFFECTOR SYSTEMS VI. Tot NEUROMUSCULAR STRUCTURE OF SEA-ANEMONES............ 76 VII. Nervous TRANSMISSION IN SEA-ANEMONES.........4.0.02000006 89 Vid. QUASI TS tS kei ene Sa eee Ra IRIN anh BN PERE ESP oe Rr ered Ae UR or 102 eee Were DRY HN Ts oo. Geof es Shige mt Aageloteia kis ert oils Mooi ayes 115 [Xo RANSMISSION IN ‘PHm NERVE=NET oo. 5.s o00c cod Moe ee warstoe an 130 XI. APPROPRIATION OF Foop AND THE NERVE-NET..............000: 138 XII. Orper Compitex RESPONSES AND THE NERVE-NET.............. 150 DLT [ERS STE Ae Co bee RM EET SS 5 ex ROM ei WRN ARF 175

SECTION III.—CENTRAL NERVOUS ORGANS Safa

ILLUSTRATIONS

FIG. PAGE 1. Diagram of the Primary Sensory and Motor Neurones of the Spinal RL GL WERUBRTATOO. .o5. © yond Ce Kate Cae ee eae 17 2. Diagram of the Primary Sensory and Motor Neurones of the Ventral Ganglionic Chain of the Earthworm................00.00000% 18 3. Ectodermic Layer from the Tentacle of a Sea-anemone............ 20 4. Diagram of the Canal System of a Calcareous Sponge...........-. 26 5. Radial Portion of a Transverse Section of Stylotella................ 30 SmMingram ol a Pinger of Sivloteda. J... <0. oe b acs bss cleligee se aera 32 7. Dermal Membrane Over a Subdermal Cavity in the Sponge Stylotella. 34 Beenited: Dernrmal.Pores 11 Sti/lotella... «Sess pc ode as oe ee 35 9. Fingers of Stylotella in a Strong Current of Seawater..............- 40 10. A Finger of Stylotella Protected from a Gentle Current of Seawater.. 41 11. Diagram of the Apparatus for Measuring the Strength of a Current of Seawater Produced by Stylotella..... 2... cccccsceenseecesness 44 12. Diagram of the Apparatus to Test the Strength of the Dermal Pores RIESE LOLI ele cs ae gia SS cae errant re Cee ene: 47 ame agra elvtne Heart, Of a ISH: 66 6:10. ocktcdin tre oo cokiveen mon: 54 14. Diagrammatic View of a Longitudinal Section of Scalpa; Enlarged Wiew oman Isolated Heartiot Scalpo ts cas tees sei) Aes +s iste ouelaee 57 15. Side View of the Ctenophore Mnemiopsis leidyi............-+-+4+- 67 PEE AN OL ste U CII ETA GUTS. SOX fers yes a 5 Asteria eile dings eee pahe ehenelapnct sca alas 69 17. Diagram of a Longitudinal Section of the Sea-anemone Metridium... 77 18. Transverse Section of the Sea-anemone Metridium................. 78 19. Diagram of the Ectoderm of a Sea-anemone..............-.+++5+: 80 20. Diagram of a Partial Longitudinal Section of the Sea-anemone VE CRAUUN DB x ad Dae Aes AMIN! ay Meine SU ato ak ovspacil sg Tonal age ky ee oe 85 21. Metridium With Its Column Completely Girdled by a Cut that Passes Stall Poms. butirely Phrough the Walle 2.5 3. cnc. yas sees 91 22. Metridium in Whose Column Wall an Oblong Incision Has Been Made so that the Resulting Piece is Attached to the Animal Only DystheVicsemberiesen, vacieie sore sous flsieeis fie sider vein ptawtis 92 23. Metridiwm Cut Vertically in Two Except in the Region of the Lips.. 92 24. Metridium from Which an Equatorial Tongue Has Been Cut from the

. Metridium from Which a Tongue Running from the Pedal Disc to the

Equator of the ColumntHas Been Cutie sec acts «550 sl'ere ao 94

. Metridium from Which a Portion of the Pedal Edge of the Column ase Beene bar blya ue, Cite was ae aichneere yl oa toss thesis oie aes (6 94

FIa.

27. Metridiwm Cut Through Vertically Except for Its Oral Disc........ 95 28. Metridium Cut Through Vertically Except for Its Aboral Disc...... 95 29. Metridium Cut Through Vertically Except for a Small Connecting

Bridge on the Pedal Edge of the Column..................-+- 96

. Section at Right Angles to the Sphincter of the Bell of Rhizostoma.. 105 . Diagram of the Jellyfish Aurelia from Which the Central Mass and

Seven of the Eight Marginal Bodies Have Been Removed and

the Outline of the Bell Further Complicated by a System of

Interdigitated Cults fcc center oie raw ocala ee ae eee 107 Diagram of the Jellyfish Aurelia from Which the Central Mass and

Seven of the Eight Marginal Bodies Have Been Removed and

the Outline of the Bell Further Complicated by a Circular

TIVCIS1ON Sarg by ecw sce sates arE ere eo el eae re eee a ee ae 108 . Diagram of a Portion of the Bell of Rhizostoma................... 110 . Preparation of Two Muscle Fields from Rhizostoma................ 111

. Preparation of Two Muscle Fields from Rhizostoma, One With a Marginal Body and the Other Without Such a Body.............. 113

. Nerve Network from a Small Blood-vessel in the Palate of the Frog. 118

. Diagram of the Nervous Organization in the Intestinal Wall of a Vertebrateicnscsdae o2 cad deny vm > dad cere ie ee cece ee 128

. Diagram of the Reactions of a Tentacle of a Sea-anemone to Tran- SECTION ha cA seks cence ote heroes etal a's > aetct he hei eheroice ca ete eee tae 131

. Sense Cells With Their Attached Fibers in a Tentacle of the Sea- BANEMONS Cemanshus: ccases = oc greeter tieelsree seine eee 132

. Diagram to Illustrate the Preparation of the Intestine in Mall’s *:_ Experiment on Intestinal Polarization. ................-.+-5- 134 . Outline and Transverse Section of the Sea-anemone Metridium..... 1386

. Diagram of a Jellyfish Aurelia Whose Bell Has Been Deeply Incised in 8 Radial Direction at Hight Places.. ......sscem pean en eee 166 . Side View of the Hydroid Corymorpha .....5.......20..csececcses 176 . A Polyp of Corymorpha Split Lengthwise Except at Its Aboral End . 188 . Diagram of an Independent Effector..................-...eee0es- 200 . Diagram of a Simple Type of Receptor-effector System............ 201 . Diagram of a Complex Type of Receptor-effector System. ......... 202

. Transverse Section of the Ventral Nerve-chain of the Marine Worm SEGGULON conse ly st 2.0 bya oer CrePegt tonsa. Wetec RT aL ee onene eS 203

. Transverse Section of the Ventral Nerve-chain of the Earthworm AllolobODhOra acuc oss WARS cv Cn CU ene ee 205

. Diagram of a Transverse Section of the Spinal Cord of a Vertebrate (Salamander) s..:c.ci< «8 ewsieincen as oe ocr iat alee 206 . Stages in the Differentiation of Sense Cells.................-0000e 210 . Stages in the Differentiation of Nerve Cells.............0..0000-- 211

. The Appropriation of Secondary Sensory Cells by Primary Sensory iINeurones in the Vertebrates csc... cue cen camer entree eine 213 12

Introduction

Tue elementary nervous system is that type of nervous system in which the structural and functional elements present themselves in their simplest states. It is most extensively represented in the lower multicellular animals, but it is found locally in as highly differentiated forms as the vertebrates. It probably represents the primitive nervous system in that it reproduces much that must have been characteristic of nervous organs in their earliest stages of evolution. It has been an object of investigation and interest only within comparatively recent years. Its derivative, that complex aggregate of organs known as the central nervous system of the higher animals, attracted the attention of anatomists and physiologists at a much earlier period. This was doubtless due to the unusual development of the central nervous organs in man and other higher animals. The human brain is a structure whose size and position naturally compels attention, whereas the chief nervous organs in many of the lower animals are often so insignificant as to be easily overlooked.

Although the first attempts at elucidating the structure and functions of the nervous system were made on the higher animals, it was a long time before this system was recognized as the exclusive seat of the most striking characteristic of man, his mental life. Only gradually was it discovered that his conscious states represent the action of a single system of organs, the nervous system, as contrasted with the rest of his body.

To the ancients this conscious life, whose chief attribute is personality, seemed to permeate the whole human frame. In fact, Aristotle, who was such an ac- - curate observer and profound thinker in so many fields of biology, denied positively that the brain was in any direct way concerned with sensation and declared the heart to be the sensorium commune for the whole body. To Galen is ascribed the belief that the brain is the seat of the rational soul, the heart the location of courage and fear, and the liver that of love. The distribution of the elements of personality over the physical body finds its expression in the common speech of to-day, particularly in relation to the heart, which is widely accepted by the popular mind as the seat of the more tender emotions. Although this opinion may commonly imply a certain amount of poetic license, it is quite certain that many an untrained person holds even at the present time to a literal interpretation of the ancient view of the location of sensations. The pain of a pin nrick is commonly believed by many persons to be where the pin enters the skin. To them nothing seems more obvious and certain than that the punctured spot is the seat of the pain, and any attempt to change their views on this point will usually be regarded by them with suspicion and mistrust, for it seems contrary to common sense,

Nevertheless, it is well known that if a nerve distributed to a given area of skin is cut at some distance from

INTRODUCTION 15, that area, the spot, though unaffected by the operation in any direct way, will give rise to no further sensations even when it is severely injured. Hence, it is clear that the pain does not reside simply in the skin. But not only may pain thus be absent from a given area of skin; it may be present when the skin with which it is supposed to be associated is absent. Persons who by accident or otherwise have lost an arm or a leg often experience long after the loss vivid and intense sensations from definite parts of the missing member. So precise and sharp are these sensations and so certainly do they seem to be associated with the lost part that some of the less knowing of these unfortunates have attempted to exhume or otherwise get possession of the lost member in an endeavor to alleviate their unpleasant sensations.

These misunderstandings, for such they are, can be swept away at once and the matter put in its true light when we recognize that our sensations are not located in the peripheral parts affected, but in the central nervous system, and within that portion of it known as the cerebral cortex. As long as this organ is intact, sensations may arise, and, though these are usually due to nervous impulses from the sensory surfaces, they may be called forth by an internal stimulus as well. Thus it is that a missing arm may be represented by sensations years after it has been severed from the body. With a loss of an appropriate part of the cerebral cortex, however, comes a loss of sensation that is absolute and final. From this there is no recovery (Parker, 1916 b).

This modern view of the relation of sensation to the nervous system was initiated by the anatomists and physiologists of the Renaissance. Thus, Vesalius taught, in the sixteenth century, that the chief soul was engendered in the brain by virtue of the powers of the proper material and form of that organ. And, although Stahl attempted, a century and a half later, to revive the belief that the soul and the sensorium commune were diffused over the whole body, that is, were resident as well in the tip of the finger as in the brain, the idea of the localization of these active properties in the nervous system became so well grounded through the investigations of the physicians of that time, particularly Haller, as to assume the form of permanency. This growth of knowledge led directly to the modern view that personality is, strictly speaking, a property of the nervous system and is in no true sense the direct result of any other system of organs. ~ (Foster, 1901.)

The nervous system, to be sure, is embedded among the other organs of the body, and the environment thus provided influences profoundly its condition and action; but what is meant by individual personality, acuteness or dulness of sense, quickness or slowness of action, temperamental traits, such as a gloomy or bright disposition, incapacity, shiftlessness, honesty, thriftiness, or sweetness, are all in the strictest sense functions of the nervous system. Hence, it is a matter of no small biological interest to ascertain from the conditions presented in the simpler animals which of these various states are elemental and what has been the probable line of evolution of that system of organs with which our personality is so indissolubly connected (Parker, 1914 a).

The nervous system of the higher animals, though enormously complex in its organization, is composed of relatively simple cellular elements, the neurones, arranged upon a comparatively uniform plan. This plan is well exemplified in the spinal nerves and spinal cord of the

INTRODUCTION 17 vertebrates (Fig. 1). In this complex the sensory neurones, whose cell-bodies lie in the dorsal ganglia, extend from the integument through the dorsal roots to the gray matter of the cord. Motor neurones, whose cell-bodies are situated within the gray matter of the cord, reach from this region to the muscle-fibers which they control. These two classes of neurones would seem to be sufficient for all ordinary reflex operations, but the cord contains within its limits other neurones which serve to connect a)

mf

1.—Diagram of the primary sensory s and motor m neurones of the spinal cord of a ceriabraia showing their connections with the integument i and with muscle fibers mf.

one part of its structure with another. These neurones, which have been called internuncial neurones, are interpolated between the sensory and motor elements just described and must thereby lengthen and extend the courses of the reflex impulses. Such neurones make up a large part of the substance of the cord and doubtless increase enormously its internal connections. In the brain they not only add to the nervous interrelations, but they afford in the region of the cerebral cortex the material basis for all intellectual operations.

The plan of neuronic arrangement as exemplified in the vertebrates also obtains in animals as lowly organized as the earthworm (Fig. 2). In this form the sensory neurones, whose cell-bodies are situated in the integument instead of being gathered into special ganglia, extend, as in the vertebrates, from the skin to the central nervous organs, the brain or the ventral ganglionic chain. The motor neurones are essentially duplicates of those in the vertebrates in that their cell-bodies lie within the central organ whence their fibers extend to the appropriate

_ Fic. 2.—Diagram of the primary sensory s and motor m neurones of the ventral ganglionic chain of the earthworm showing their connections with the integument i and with the muscles ms. (Modified from Retzius.)

musculature. Internuncial neurones are also abundantly present in the earthworm, though their function here, in contrast with that in the higher vertebrates, is pure nervous intercommunication, for it is very unlikely that the earthworm possesses what in any strict sense of the word can be called intelligence. Thus from a morphological standpoint, the nervous systems of the higher animals, even including such forms as the earthworm, have much in common, their three sets of interrelated neurones, sensory, motor, and internuncial, being arranged upon what is in the main a uniform plan.

Considered from a physiological standpoint, the nervous system with its appended parts as just sketched falls in the higher animals into three well-marked categories. On the exterior of these animals are to be found sense organs or receptors such as the free-nerve terminations

INTRODUCTION ute)

of the sensory neurones in the vertebrates or the sensory cells in the integument of the earthworm. These organs have for their function the reception of the external stimuli and the production of the sensory impulses. The receptors are connected by nerve-fibers with the central nervous organ or adjustor composed of the central ends of the sensory and the motor neurones and of the internuncial neurones. Here the impulses arriving from the receptors are directed toward the appropriate groups of muscles by which the animal may respond to the stimulus and, if the animal is highly organized, impressions are made upon the adjustor which, as memories, may become more or less permanent parts of the animal’s nervous equipment. Finally the adjustors are connected by nerve-fibers with the third set of elements, the effectors, which as muscles, electric organs, glands, etc., enable the animal to react on the environment. Thus three physiological categories are to be distinguished which in the order of their sequence in action are sense organs or receptors, central nervous organs or adjustors, and muscles or other effectors.

It is to be noted in passing, that the physiological scheme just outlined includes a wider range of parts than is generally admitted under the head of the nervous system. The additional parts are the effectors, which, as will be shown later, form as truly a part of the whole system as do the sense organs or the central nervous organs. Since the term nervous system does not ordinarily include the effectors, it is perhaps best to designate the whole chain of related parts, receptors, adjustors, and effectors, as the neuromuscular mechanism, and in dealing with the elementary nervous system it will be found important to keep this relation in mind, for in such an inquiry, the real question that must be confronted concerns the interrelations within the neuromuscular mechanism rather than those that are simply within the nervous system itself.

The type of neuromuscular mechanism described in the preceding paragraphs in which a group of receptors is connected with a well centralized adjustor that in turn controls a complex system of effectors, is found only in the more differentiated animals. Certainly in the simple forms, like the jellyfishes, corals, sea-anemones and so forth, only the slightest evidence of this type of nervous organization can be discovered. Nevertheless, these animals possess a neuromuscular mechanism, but on so simple a plan that investigators have long been inclined to regard it as representing the first step in the

differentiation of neuromuscular organs. Fia. 3.—E ot o- P ° dermic layer from This plan of structure is well represented the tentacle of a sea-anemone show- in the sea-anemones. Hach of the two laylayers, epithelial ¢ ers of cells that make up the living sub- SUES AR Ry stance of the sea-anemone’s body consists ordinarily of three sublayers; a superficial or epithelial layer, a middle or nervous layer, and a deep or muscular layer (Fig. 3). The epithelial layer contains, besides many other kinds of cells, large numbers of sensory cells which terminate peripherally in bristlelike receptive ends and centrally in fine nervous branches. These fine branches constitute collectively the middle or nervous layer in which occasionally large branching cells, the so-called ganglionic cells, occur. Immediately under the nervous layer is the deep layer of elongated muscle-cells.

INTRODUCTION 21

The condition thus briefly described is present over much of the sea-anemone’s body, and though the nervous layer may be somewhat emphasized in some regions, it cannot be said to be really centralized in any part. Hence this type of nervous system has been designated as diffuse in contrast with the centralized type found in the higher animals.

What is really present in the neuromuscular portion of the sea-anemone’s body is a large number of peripheral sensory cells whose deep branching ends connect more or less directly with the muscles, that is, without the intervention of a true central organ. This neuromuscular system, if described in the terms already used, could be said to be composed of receptors and effectors without an adjustor or at least with this member present in only a most undeveloped state. Hence the adjustor or central organ is in all probability an acquisition that represents a later stage in the evolution of the neuromuscular mechanism than that seen in the celenterates.

If the celenterates represent a stage in the development of the neuromuscular mechanism in which sensory cells and muscles are the only important parts present, it is natural to ask if there is not a still more primitive state from which the celenterate condition has arisen. On this question several hypotheses have already been advanced. Claus (1878) and, subsequently, Chun (1880) maintained that originally the nervous system and the muscles were differentiated independently and that they became associated only secondarily. This view has deservedly received very little attention, for it is extremely difficult to conceive of an animal that would develop receptive ability without at the same time acquiring the power to react. Such an animal would have a certain resemblance to a person suffering from complete motor paralysis who might still be able to receive impressions from the exterior and even to reflect on them, and yet would be incapable, in consequence of the complete separation of nerve and muscle, of carrying out activities that would maintain a harmonious adjustment with the exterior. Not only does this view of the independent origin of nerve and muscle meet with the inherent and serious difficulty just alluded to, but among the lower forms not a single animal is known in which nerve is unassociated with muscle. Hence the hypothesis of Claus and of Chun has received very little serious consideration.

Much less subject to criticism than the hypothesis of the independent origin of nerve and muscle is Kleinenberg’s theory of the neuromuscular cell. In 1872 Kleinenberg announced the discovery in the fresh-water hydra of what he designated as neuromuscular cells. The peripheral ends of these cells were situated on the exposed surface of the epithelium, of which they were a part and were believed to act as nervous receptors; the deep ends were drawn out into muscular processes and served as effectors to which transmission was supposed to be accomplished through the bodies of the cells. Each such cell was regarded as a complete and independent neuromuscular mechanism, and the movements of an animal provided with these cells was believed to depend upon the simultaneous stimulation of many such elements. It was Kleinenberg’s opinion that these neuromuscular eells divided and thus gave rise to the nerve-cells and muscle-cells of the higher animals. In fact, he declared that the nervous and muscular systems of these animals were thus to be traced back to the single type of cell, the neuromuscular cell, which morphologically and physiologically rep-

INTRODUCTION 23 resented the beginnings of both. But Kleinenberg’s neuromuscular cells were subsequently shown by the Hertwigs to be merely epitheliomuscular cells and no intermediate stage between them and the differentiated neuromuscular mechanism of higher forms was ever discovered. Hence this hypothesis, too, has been largely abandoned (Parker, 1911).

Some years later, in 1878, the Hertwigs published an account of the neuromuscular mechanism in ccelenterates. In this account they described the sensory cells, the ganghonic cells, and the muscular cells of the ccelenterates, and maintained that these elements arose not by the division of single cells, as stated by Kleinenberg, but that each element was differentiated from a separate epithehal cell and yet in such a way that during differentiation all these elements were physiologically interdependent. This hypothesis of the simultaneous differentiation of nerve and muscle, which has been the current opinion among biologists for more than a generation, is not without its serious difficulties, for it appears that in the sponges, which are more primitive animals than the ceelenterates, there are muscles of a very simple type but without any associated nerve (Parker, 1910 a). In fact, no nervous tissue of any kind has been definitely identified in sponges. It, therefore, appears that of the two elements, nerve and muscle, the latter.may exist independently of the former and in such a way as to indicate its more primitive character. From this standpoint it seems that the receptor-effector system of the cclenterates was preceded by a simpler state in which only the effector element, muscle, was present and that this element may, therefore, be regarded as the original one in the evolution of the neuromuscular mechanism. Muscle once developed as an effector gave occasion for the addition of receptors or sensory elements after which the adjustor or central nervous mechanism was differentiated. Hence in dealing with the elementary nervous system it will not be inappropriate to consider first such independent effectors as are found in sponges after which the receptor-effector system of the celenterates may be dealt with, in part as a system in itself and in part as the source of the differentiated receptor-adjustor-effector systems of the more complex invertebrates and the vertebrates. Heretofore the approach to this subject has been chiefly from the side of anatomy and histology; but important as this line of research has been, the more recent experimental results have shown that the physiological side of the question is quite as illuminating as the morphological. In the following pages some of the more important problems of the elementary nervous system will be diseussed. While these problems will be taken up chiefly from a functional standpoint, the structural aspect of the questions concerned will not be omitted, for in disecussions of this kind the double viewpoint is more likely to lead to sound conclusions than if only one aspect is kept in sight (Parker, 1909, 1910 b, 1914 b).

Section I. Effector Systems Chapter II Sponges

SponcGzEs are such inert organisms that their membership in the animal series was for a long time unsuspected. Their methods of development, however, settled this question beyond dispute. In form they are either single, more or less goblet-shaped animals (Fig. 4) or somewhat amorphous colonies whose chief activity is exhibited in the very considerable currents that they produce in the surrounding water. These currents enter the substance of the sponge through innumerable pores scattered over the surface of its body, pass through its more solid parts by a system of canals that converge on a central cavity, the cloaca, from which they emerge by a large opening, the osculum. This opening is situated at the apex of the individual sponge or, in the case of a colony, each osculum is ordinarily on a somewhat conical elevation rising above the general surface of the colony.

The currents that emerge from sponges are often so considerable as to deform the surface of the water above ' them much as is done by a vigorous spring. They are produced by the action of the choanocytes or flagellated cells that line the canals in the body of the sponge or that invest more specialized chambers interpolated on the course of these canals. So far as can be judged by the examination of the inner parts of sponges, the choanocytes are incessantly active, and the obliteration and revival of currents as seen in many of these animals are due not to changes in the activity of the choanocytes but to other causes.

Some sponges, such as the fingered form Stylotella, appear, when out of water, to be more or less shrivelled or contracted and under other circumstances to be plump and well rounded-out. The differences, which, for reasons to be mentioned presently, are known not to be due to the simple physical loss of fluid, are apparently dependent upon a general contractility of the whole flesh of the sponge which, though slight, may nevertheless enable the sponge to change its form somewhat. Aristotle in the fourteenth chapter of his fifth book on the history of animals

makes the interesting state- Fig. 4.—Diagram of the canal system of a calcareous sponge (modified ment that the sponge 1S sup-

Haeckel). The innumerable superficial pores receive water from the exte- posed to possess sensation rior, as shown by the arrows on the . ca sides; the osculum at the apex dis- be -rause 1t contracts if it percharges water to the exterior

| eceives any movement to tear it up and it does the same when the winds and waves are so violent that they might loosen it from its attachment. He further adds in his characteristic way that the natives of Torona dispute this.

The idea that the common flesh of the sponge is contractile is not without modern support. Merejkowsky (1878) stated that if the sponge Suberites is so placed that it is partly out of water, it will curve the body until it is under water as much as possible, and if the body is then covered with water, it will return to its former position.

It must be evident, from what has already been stated, that much of the common flesh of Stylotella is contractile. As already noted, specimens out of water quickly assume a shrivelled and rugose appearance as though the flesh had contracted on a resistant skeleton, a condition that it also quickly assumes in quiet seawater. Moreover, if a sponge is placed partly in running seawater and partly in the air, the portion in the seawater remains smooth and that in the air becomes rugose. Specimens made rugose either in the air or in quiet water soon recover their smooth appearance on being placed in running seawater. Air or quiet water may then cause a contraction of the common flesh of Stylotella, a condition counteracted by running water.

The contraction of the common flesh can also be well seen around some of the larger cavities, such as the cloacal cavity. Ifa long finger of Stylotella, whose two ends have been cut off and whose cloacal cavity extends along one of its sides, is placed in quiet seawater, the cloacal cavity is soon indicated by an external groove due apparently to the partial collapse of the cavity. This groove, however, is caused not by collapse, but by the contraction of the common flesh which as partial partitions or even trabecule is abundant about the sides of the cloaca. On returning the finger to running water the flesh relaxes and the groove mostly disappears.