Comparative physiology for London University matriculation and science and art examinations

Part I. London

W. STEWART & CO., IIOLBORN VIADUCT STEPS, E.C. EDINBURGH : J. MENZIES & CO.

fl % - . i byn^ in

h

o

w

w

p

in h

X

w

in

H

Pi

W

h

LONDON: W. STEWART & CO., HOLBORN VIADUCT STEPS, E.C.

Respiration — continued, ...... 100

COMPARATIVE PHYSIOLOGY.

Introduction

OF late years there has arisen in the minds of not a few some little confusion as to the absolutely exact meaning of the term Biology. It may be well to point out clearly the two different ways wherein the word is used. Reference to the subjoined table may aid the reader in the comprehension of what follows.

/(rt)Vegetable=Botany j Physiology. t^o^P^^oLgy. I ( Classification. '

(Anatomy, .

{b) Animal=ZooIogy. ■< Physiology. ^

(Classification.

Biology

Biology is, as far as definition is dependent on derivation, the part of Science dealing with all living things — with all things possessing the power of accommodating their internal conditions to the ever-varying conditions of their environment. Of these living things there are two distinct forms — Plants and Animals. Hence, without in any way attempting in this place to distinguish these two great groups of organized beings, we can divide

A

COMPARATIVE PHYSIOLOGY.

Biology into {a) Biology in reference to plants, (/^) Biology in reference to animals. To the former a distinct name is given, Botany. To the latter many give the name Zoology, regarding that science as parallel to Botany, and covering similar giound to that which Botany passes over.

In studying plants there are three classes of facts to be acquired. For example, we can see (i) that the root of a bean plant has a main central axis running down into the ground, with certain small secondary branches. (2) We can learn that the root takes in material from the soil, and gives out salts to the soil. (3) We can discover that all plants with axial roots have leaves with a network of veins and two cotyledons in the embryo. The first of these facts is a structural one. It falls under the province of Anatomy. The second relates to function. It falls under the province of Physiology. The third is connected with the classifying or grouping of plants. To- one or other of these three facts every statement in relation to plants can be allied. Under one or other of these three divisions of Botany — Anatomy, which tells us what plants and parts of plants are ; Physiology, which tells us what plants and parts of plants do ; Classification, which tells us of the relationship between plants — every fact in Vegetable Biology must come.

Turning to the study of animals, similar series of facts are met with. We study the structure of animals (Anatomy), their functions (Physiology), their grouping (Classification).

In the study of the struchire of living beings, two great classes of facts present themselves. We can acquire knowledge of the larger, easily-seen parts or organs of plants or animals. We can study the shapes of leaves, the number of members in the

INTRODUCTION.

whorls of a flower, the arrangement of the cavities of tlie heart, the divisions of the alimentary canal. This is Morphology, or the study of organs. But we can do more than this. With the aid of a knife, microscope, chemical reagents, we can dissect organs, can see their minute structure, can study their very penetralia. This study of tissues is Histology. Anatomy = Morphology H- Histology.

Biology, then, should include everything known as to the structure, the functions, the grouping of all plants and of all animals.

But of late the word Biology has been somewhat restricted in its meaning. Professor Huxley and the Senate of the London University are answerable for this. These are great authorities, whom it behoves us to follow reverently. Professor Huxley in his work on Elementary Biology, and the Senate of the London University in the Syllabus of the First B.Sc. Examination, seem, as far as we are able to understand them, to include under the name Biology —

{a) The study of the lower forms of both vegetable and animal life ;

{b) The study of the typical forms of both vegetable and animal life.

Thus, in Professor Huxley’s book, he leads the student to the investigation of

Yeast or Torula.

Protococcus.

Proteus animalcule (Amoeba).

Bacteria.

Moulds.

Stoneworts.

Bracken (Fern).

The Bean Plant.

Infusoria.

Hydra.

Fresh-water Mussel, Crayfish.

Frog.

As to the University, the list is much the same, but there is added an account of the principal tissues of the animal body.

COMPARATIVE PHYSIOLOGY.

It is perhaps open to question whether this later arrangement is really a philosophical one. In the study of Biology there are few more important laws than that of Von Baer, — the law of progress from the general to the special. In the lower plants and animals, all functions are performed by all tissues. The cell wall and contents of a Torula, the sarcode of an Amoeba, assimilate, breathe, reproduce. No special part is set aside for the functions of digestion, of respiration, of reproduction. But as we ascend in the scale of living beings, we find particular parts of the plant or animal body reserved for special actions. Axis becomes distinct from appendages ; stem begins to transmit, leaves to breathe in the plant. Mouth and alimentary canal are marked off from the rest of the body ; certain definite parts of the surface take on themselves the function of breathing in the animal. Differentiation progresses, until in the performance of ^very function there is a definite region of the body concerned.

Von Baer’s law holds generally in scientific nomenclature. A few years back, the general name Natural History was much in vogue; and when a man studied minerals, plants, and animals, he was a student of Natural History. By degrees, however, differentiation took place, and men began to turn their attention specially to Geology, or to Botany, or to Comparative Anatom.y. A curious transition stage between the old Natural History and the present three great sciences ^each whereof is gradually undergoing further division) is shown by the use of the words Natural History in the preface to Huxley and Martin’s Elementary Biology. Therein Natural History is said to be synonymous with Biology, with the study of living beings only.

INTRODUCTION.

The present use of the term Biology seems to be the result of a reverse action to that so constantly occurring, the verbal expression whereof is the law of Von Baer. We seem here to have passed from more special names to a more general one.

In favour of the same transition, it may be urged that the aim of Science is to arrive at larger generalizations, and that the substitution of the single word Biology for the two names Botany and Zoology, is an advance : that under Biology we are to range all wide facts that are true of all living things. It must not be forgotten, however, that we are obliged still to retain the sciences of Botany, of Comparative Anatomy and Physiology, and of Zoology. More abstract, however, than ail these, an embodiment of generalizations founded on the facts of all these, is Biology.

It is not possible to study one branch of Biological Science without acquaintance with others. In the following pages an attempt will be made to systematize the known facts in relation to the physiology of animals. The main object of study will be Comparative Physiology. But, as it is not possible to investigate a function without a knowledge of the organ performing it, Comparative Anatomy will also be studied ; and as special animals and special classes will have to be referred to, the leading facts in the classification of animals must be discussed more than once. For the full comprehension of facts in regard to animals, the structure and functions of plants must be spoken of, and the domain of Botany will be trenched upon. Especially will the understanding of the chief principles of classification be necessary. A table is therefore subjoined, showing the system to be used in this book ; —

ANIMALIA.

Sub-Kingdom. Class,

{Gregarinida, .

Rhizopoda, . .

Infusoria, . .

f Porifera, . .

CcELENTERATA, . Alfydrozoa, . .

\AcHnozoa, . . f Polyzoa, . . .

jC Atriophora, < Ascidioida, . .

( Brachiopoda,

MOLLUSCA,

Branchio- ^ phora,

Annulosa, -

^Vertebrata, . . ,

^ Cephalopoda,

{Scolecida, . .

Echinodermata, Gephyrea, . .

Annelida, . ,

{Arachnida, . .

Crustacea, . .

f Pisces, . . .

Amphibia, . .

Reptilia, Aves, . . .

^Mammalia, .

Example.

Gregarine.

Amoeba, Foraminifera.

Vorticella.

Sponge.

Hydra.

Sea Anemone.

Filustra.

Ascidia.

Terebratulau

Oyster,

Snail.

Haliotis.

Cuttle-fish.

Tape-worm.

Echinus.

Sipunculus,

Earthworm.

Spider.

Centipede.

Fly.

Lobster.

Salmon.

Frog.

Snake.

Pigeon.

Horse.

All our facts and theories will tend to the great ends of gaining generalizations as to all living things, of arriving at statements that can be predicated of all plants and animals, of building up a true science of living things.

It has been said that the verbal expression of one of the most important generalizations in Biology is called Von Baer’s Law. It is the law of ‘progress from the general to the special.’ All functions of the lowest animals and plants are performed indifferently by all parts of the body. No special regions are marked off for special functions. As we ascend in either the Vegetable or the Animal Kingdom, certain parts of the body become marked

INTRODUCTION.

off for particular functions. One area is restricted to Digestion, another to Circulation, a third to Reproduction. The more highly organized the being, the more complete is the division of its body into distinct organs, each with a special action. It is designed in this volume to trace out, in relation to the Animal Kingdom, the gradual steps whereby any given function, at first performed indifferently by any portion of the body, becomes restricted to particular portions with an ever-increasing specialization of the organs concerned in it.

The order wherein the various functions and organs will be considered in the following pages is as follows : — (i) Digestion, or the preparation of the food. (2) Absorption of digested food. (3) Circulation of the blood. (4) Respiration, or the purification of the gases of the blood. (5) Secretion, or the separation of liquids and solids from the blood. (6) The Nervous System. (7) The Sense Organs. (8) The Motor Organs. It will not be possible within the limits of this book to discuss the advance in specialization in the functions of Reproduction and Development.

Chapter I. Digestion In Protozoa And Coelenterata

The first function to be considered is Digestion.

When it is remembered that we have to trace the history of digestion upwards from its most general form, it will be seen how difficult it is to give any definition of the word to include all cases. Only an approximation can be hoped for. Such an approximation may bb found in the statement that digestion is the process whereby food is taken into the organism and made fit to become part thereof.

I. Protozoa — (i) Gregarina. — This animal is found infesting the interior of certain invertebrates, such as earthworms and cockroaches. It absorbs food by every part of its surface, and probably gives out material at every part of its surface. Looking at these facts, and at the fact that the food so absorbed is in some cases at least already partially digested by the animal in which the Gregarine is resident, this may be regarded as one of the simplest cases of digestion in the Animal Kingdom. It may be well to state here, once for all, that we shall not find the gradual specialization of different functions following the same lines in the animal series. Nor will the advance of particular functions keep pace with the advance in

DIGESTION.

structure. In absolute structure, for instance, the Gregarine ranks higher than some of the beings referred to below.

(2) Amoeba. — This well-known jelly-like Rhizopod, met with in the waters of ponds and aquaria, thrusts out from any part of its body extensions known as pseudopodia towards any food particles that may be in its vicinity, seizes them, and then retracting its pseudopodia, draws them into the substance of its body, and with them the food particles. Within the body, the food thus taken in has its nutritive matter gradually extracted by the protoplasm or sarcode, whereof the creature consists j while the indigestible residue passes out of the body at any point. It is to be noted here, that any part of the body may be protruded as pseudopodia, that the food particles in passing through the sarcode take no definite course, that the portions of them not absorbed are extruded at no definite region. Yet here is an advance upon the Gregarine, as a part of the body is extended into pseudopodia, but it is a matter of indifference which particular part. A mouth region and an anal region are marked off for each particular particle of food, but there is no mouth and there is no anus.

(3) Pamphagus. — A Rhizopod of similar structure to the Amoeba, but the pseudopodia are given out from one particular region only of the body. So is it with Dififiugia and Arcella, two simple Rhizopods that di.Ter from Amoeba and Pamphagus in having an external covering. The pseudopodia are extruded only from the single aperture present in each such external covering.

(4) Foraminifera. — These complex Rhizopods may be looked upon as colonies of amoebiform

COMPARATIVE PHYSIOLOGY.

creatures all connected with each other, and surrounded by a complex shell. Two phases of digestion present themselves here : (a) Each division of the shell is perforated by many apertures, through which the pseudopodia of the particular amoebiform creature dwelling within that division are protruded. (^) The shell is not perforated, but the food for the whole colony is seized and taken in by pseudopodia given off by the individual segment found in the last-formed and therefore most free cavity of the shell.

(5) Infusoria. — The well-known inhabitants of water in which vegetable matter has been for any length of time show a very distinct advance. There is in these, for the first time, an absolute and constant aperture — the mouth. Towards this, food particles are swept by the action of the cilia, or delicate, structureless, vibratile extensions of the body. Food entering the mouth passes down a distinct tube — the gullet or oesophagus. This oesophagus does not, however, dilate into a stomach, nor indeed are there any other parts of the alimentary canal except mouth and oesophagus. The latter is closed below, or rather clogged by the sarcode of which the body still consists. Through this sarcode passes the food that has been ingested ; but it has a very definite shape, and passes in a very definite direction. At the bottom of the oesophagus the food is shaped into pellets, each surrounded by a layer of water. These are called food vacuoles, the watery part giving them the appearance of partially empty spaces. The older food vacuoles, as new ones are formed, are pushed on into the interior of the body. The direction in which they move is, as has been said, definite. Each is driven on by its successors

DIGESTION.

II just beneath the outer layers of the body, and so makes a complete circuit of the Infusorium. The food pellets become smaller and smaller the longer they have been in the body, and, finally, the indigestible residue passes out in a similar fashion to that met with in the AnuDeba. There is no actual opening ; but there is a very distinctly-marked region where this extrusion occurs. Here, then, is a mouth, oesophagus, a distinct course for the food, though there is no intestine, and an anal region though no persistent opening.

2. CcELENTERATA — (i) Hydra. — This being, found on the roots of duckweed, is a simple bag fixed by the base and open at the other extremity. The opening, which is surrounded by tentacles, is the mouth, and leads into a single cavity. This single cavity in the interior of the creature is at once the general cavity of the body, or somatic cavity, and the digestive cavity. It may be necessary here to make quite plain what is meant by the terms ‘somatic cavity’ and ‘digestive cavity,’ as much confusion appears to exist in many minds as to the meaning of the phrases. The somatic or body cavity is the cavity contained within the general body wall, and would remain empty if all the internal organs were removed. In the higher animals this general cavity is traversed by a tube of greater or less complexity, communicating with the exterior, but completely shut off from the general cavity \ this tube encloses the digestive cavity.

In the Hydra the body contains but one single cavity. Clearly this is a somatic one ; but inasmuch as food is constantly being introduced into it by the mouth, it is also a gastric or digestive one. The two cavities, distinct in higher animals, are here fused into one. Food taken into the creature

COMPARATIVE PHYSIOLOGY.

has the nutritive portion of it absorbed by the body walls, and the non-nutritive probably ejected by the mouth. Here is a cornplete mouth and alimentary canal ; but the latter is still one with, and not cut off from, the general cavity of the body.

(2) Sea Anemone. — This animal, found in large numbers adhering to the rocks on our own coasts, resembles the Hydra in being fixed by its base, and in having at the other extremity a mouth surrounded by tentacles. It is, however, no longer a simple bag, but a double one. It consists of one bag suspended within another. The one is the digestive cavity, the other is the somatic. Food passes through the mouth directly into the digestiye cavity, which presents no traces of division of any kind. It is a simple tube. Below, this digestive cavity opens into the general body cavity in which it is suspended. In the Hydra the two cavities are one. In the Sea Anemone they are distinct from, but communicate with, each other. The hollow tentacles surrounding the mouth are elevations of the roof of the somatic cavity, and are perforated at their free apices. Food entering the mouth undergoes digestion in the suspended alimentary canal. The results of its digestion either transude through the walls of the alimentary canal into the surrounding body cavity, or pass directly from the one to the other by the aperture in the base of the former. Such portions as are not digested quit the body, not at one definite aperture, but at the perforated apices of the tentacles.

So far we have seen the function of digestion performed by the whole of the body (Gregarine) ; by any accidental portion thereof {Rhizopoda) ; by a mouth, oesophagus, and tolerably definite portion of the sarcode of the body, without,

DIGESTION.

however (after the oesophagus), any distinct tube {Infusoria)’, by a mouth and distinct alimentary canal, one with the body cavity (Hydra); by a rnouth and distinct alimentary canal suspended in, distinct from, but communicating with the body cavity (Sea Anemone).

Chapter II. On Digestion In The Mollusca And Annulosa

IT will not be advisable to deal, as in the first chapter, with a series of animals whereof the whole digestive apparatus shows an ever-increasing complexity. It will be better to take the digestive apparatus itself, and to note in what animals that apparatus shows advance in one direction, in what animals it shows advance in another. Some creatures present improvement in structure and in function along one particular line, whilst others have specialization of an entirely different part of the alimentary canal. It is therefore often difficult, or even impossible, to say positively whether the whole digestive apparatus of a given animal is of a higher or lower order than that of some other animal.

A. Alimentary Canal still communicating with the Body Cavity. — Above the Sea Anemone in the animal series, this communication between the digestive and the somatic cavities is very rare. Striking instances thereof are presented by Ascidia and other members of the class Ascidioida. The alimentary canal in these animals is sufficiently complex, but the dilated pharynx is perforated. Thus its interior is in communication with the general cavity of the body wherein the digestive

- DIGESTION IN MOLLUSCA AND ANNULOSA. 1 5

tube is suspended. Further, the intestine of Ascidioida does not open directly on the exterior of the body, but opens into a chamber known as the atrial. This atrial chamber does communicate with the exterior of the body.

B. Alimentary Canal communicating with Organs concerned in Functions other than Digestion. — Three principal cases present themselves : — {a) With the absorptive apparatus. The function of absorption in the Invertebrata is never carried on by a distinct set of vessels. In the lower Mollusca and Annulosa, the walls ol the alimentary canal absorb the digested food. In the higher, the blood-vessels in the walls of the alimentary canal absorb the digested food. In both cases, the two functions of digestion and absorption are not completely differentiated from each other. The details of the advance in complexity of the absorptive function will be worked out more fully in succeeding pages, {b) With the circulating apparatus. The organs of digestion and of circulation in some cases are in connection. The alimentary canal of Brachiopoda and that of Lamellibranchiata perforate the heart. That of Haliotis, one of the class Gasteropoda^ is embraced by but does not perforate the heart, {c) With the. respiratory apparatus, (i.) The pharynx or upper end of the alimentary canal of Ascidia, referred to above, is connected with respiration as well as digestion. It is perforated. It is richly supplied with blood-vesse’o. The margins of the perforations are ciliated, i.e. provided with very fine motile hairs or cilia. Through these apertures water, air-laden, is ever rushing. Thus the blood in the vessels of the pharynx is brought into relation with the air in the water. Here is an organ presenting a fusion of the functions of digestion and respiration, (ii.) In Rotifera or the Wheel-animalcules, the lower end of the alimentary canal has connected with it the tubes of the water-vascular or respiratory apparatus.

C. Alimentary Canal closed at one end. — In some Invertebrata the intestine is blind. A mouth or anterior opening is present. There is no anus or posterior aperture. Two orders of the class Scolecida, viz. the Trematoda, whereof Distoma is an example, and the Turbellaria, illustrated by Planaria, and the Ophiurida of the class Echinodermata, have an alimentary canal closed at the end.

D. Reversions. — Before turning to the consideration of individual portions of the digestive canal, attention must be called to certain curious reversions to a very low type of digestion, met with in animals comparatively high in the scale of being. Some Entozoa, as the tape-worm, have no digestive organs. They live, as Grcgarinida live, by imbibition of the partly digested food of the animals they infest. The males of some Rotifera are also destitute of any digestive apparatus. The whole of their brief life is devoted to reproduction.

E. Mastication. — Turning to the account of specialized parts of the alimentary canal, it is necessary to refer first to those arrangements whose purpose is the comminution of the food, the breaking up into smaller portions of the materials to be digested. The general term masticatiofi may be employed here, {a) Perhaps the least specialized form of masticatory apparatus is the gizzard, though this organ is met with in animals ranking high in the animal series. A gizzard may be defined as an exceedingly muscular stomach. The

DIGESTION IN MOLLUSCA AND ANNULOSA. 1 7

walls of all dilatations of the alimentary canal will be muscular. Let the walls of a particular dilatation be very muscular and often provided with specially hard tendinous or even calcareous or silicious regions, and the dilatation is a gizzard. Such an organ is met with in the Polyzoa^ Gasteropoda^ Cephalopoda^ Rotifera, Earthworm, and the higher Insecta. {b) Hardenings of the mouth-lining or buccal mucous membrane form a more specialized apparatus for mastication. The structures concerned in breaking up the food have now assigned to them a definite position at the beginning of the digestive tube. They are separated from the stomach. In the Annelida, as the earthworm, tube-worm, leech, the ‘jaws’ are hardened parts of the mouth mucous membrane. If they can be compared with any structure in the higher animals, it is with the teeth of the latter, which are also developed from epithelial membranes. That this is a low form of masticatory apparatus is shown by the fact that the number of ‘ jaws ’ may vary on opposite sides of the same mouth, {c) The exceedingly complex masticatory apparatus of Echinus, v/ith its five jaws, each traversed by a tooth, and all moved by an elaborate arrangement of muscles, is still, probably, only a very specialized hardening of epithelial membrane, {d) A similar statement may be made in relation to the odontophore of the higher Mollusca. This structure, with its cartilaginous support, its long elastic strap, its transverse rows of teeth, seems to combine, to some extent, the functions of teeth and of tongue, (r) The mouths of the higher Annulosa present a higher type of masticatory apparatus. Their jaws are appendages of the body-segments. They are modified limbs. (i.)The simplest condition is 4;hat

B l8 COMPARATIVE PHYSIOLOGY.

met with in the Myriapoda, as the Centipede. A small labrum above the mouth-opening, a pair ol’ mandibles or hard, crushing jaws, a labium below the mouth-opening, with side lobes, (ii.) The Arachnida (spider and scorpion) have labrum and mandibles, and two pairs of more delicate jaws, known as maxillae. These are the side lobes of the labium of Centipede specialized into distinct lateral jaws. In Arachnida, the mandibles are extended into prehensile and offensive claws. The maxillae in spider are related in a remarkable manner to the function of reproduction. The specialization is therefore incomplete, (iii.) Crustacea, the lobster, have a labrum, two mandibles, four maxillae (the second pair representative of the split labium of Myriapoda), and three pairs of feet-jaws (maxillipedes). These last represent the six legs of Insecta. The somites, which in the latter bear motor organs, carry in the Crustacea organs that serve for mastication, but that are in structure closely allied to the true legs on the succeeding somites, (iv.) In the Insecta, the labrum, labium, mandibles, maxillae, are all met with, and present numberless modifications far too complex to be even mentioned here. All these modifications are, in the main, subservient to the functions of taking in or crushing food. (/) Finally, whilst all the jaws mentioned above move in a horizontal plane, in the cuttle-fish {Cephalopoda) the jaws move in a vertical plane.

F. Stomach. — A dilatation of the alimentary canal is a stomach. No such dilatation is found in some of the Echinodermata, and a very slight one in the Ascidioida. Forms of stomach. — {a) The most necessary and constant form of stomach is an expansion of the alimentary canal, where the food is

DIGESTION IN MOLLUSCA AND ANNULOSA. 19

delayed, and whereinto digestive fluids are poured. This is the true stomach, present in the majority of Mollusca and Annulosa, with the exceptions just mentioned, (d) The Gizzard. — Reference has been made to this, (c) The Crop. — An expansion of the alimentary canal, very generally a lateral one, whereinto no digestive fluid is poured. The crop serves to store up the rapidly-swallowed food until time is afforded for that food to be passed on to the true stomach. The crop does not digest. It is seen in the higher orders of Insecta.

G. The Intestine. — The narrowed part of the alimentary canal succeeding the stomach, {a) It is straight and without convolutions in many Mollusca and in most of the Annulosa, as the Star-fish, the Scorpion, the Centipede, and even the Crustacea, (p) It presents convolutions in some. These cases are more particularly met with in the Mollusca, in which group the digestive apparatus on the whole presents a higher type than that seen in the Annulosa. Even the lower Mollusca have a flexure or bend in the intestine, {c) Lastly, the distinction between the small and large intestine, a distinction founded on their relative diameters, not on their relative lengths, is observable in the Insecta and in many Mollusca.

Chapter III. On Digestion In The Vertebrata

Pisces^ Amp/iibia, Reptilia,

The structure of the digestive apparatus in the sub-kingdom Vertebrata remains for consideration. It will be best to study the individual classes of that sub-kingdom one after another. It will be observed that, as we ascend through this .the highest group of the kingdom Animalia, we shall meet with that ever-increasing heterogeneity, and also that ever-increasing definiteness, that Mr. Herbert Spencer has shown to be the accompaniments of evolution. We shall find more parts appearing in the digestive apparatus, and we shall find each of them more and more distinctly marked off from its fellows, both structurally and functionally. Different kinds of teeth will replace a dental system whose members are all exact counterparts one of the other. The simple intestine will become differentiated into several special regions. Each kind of tooth will attack the food in the process of mastication in its own special way. Each region of the intestine will have its own distinct office in the digestion of the food.

(i) Pisces. — A. Teeth. — The teeth are nearly all conical, are distributed over the surface of the interior of the mouth and pharynx, and are not imbedded in sockets. B. Stomach. — In some fish

DIGESTION IN VERTEBRATA.

the stomach is not really distinct from the oesophagus. In the majority it is wide and sac-like, and in scarcely any is it very clearly marked off from the preceding and succeeding parts of the alimentary canal. C. Small Intestine. — Comparatively short. At its commencement are several blind tubes (caeca) opening into the intestine directly. From their proximity to the pyloric end of the stomach, they are known as the pyloric caeca. In many fish the surface of the intestine is increased by the extension of its internal mucous coat into a fold running spirally along the wall of the intestine, and called the spiral valve. There are scarcely any villi or minute elevations of the mucous coat. The spiral valve seems to represent a less specialized method of increasing the absorptive surface than the villi met with in higher classes. Lastly, the mesentery, or special fold of peritoneum slinging the intestine on to the spinal column, is much reduced compared with that of higher vertebrates, and sometimes in the peritoneum are openings rendering that serous membrane no longer a closed sac, but putting it into communication with the exterior of the body. D. Large Intestine. — Short ; only marked off from the small by a constriction and an internal valvular fold; without a caecum. In fact, the large intestine is homologous with the rectum alone of Mammalia. It ends very frequently, but not universally, in a cloaca, i.e. its final portion has opening into it the ducts of the renal and reproductive organs. The cloacal outlet or anus varies much in position. It is at times placed very far forwards in the body. E. Glands. — (a) Salivary glands wanting, (b) Pancreas. The pyloric appendages are secretory. They are sometimes spoken of as representatives of a pancreas, but