CHAPTER VIII.. The Organs of Nutrition and Digestion.

THE ORGANS OF NUTRITION AND DIGESTION.

Fishes are either exclusively carnivorous or herbivorous, but not a few feed on vegetable substances as well as animal, or on mud containing alimentary substance in a living or decomposing state. Generally they are very voracious, especially the carnivorous kinds, and the rule of “eat or be eaten” applies to them with unusual force. They are almost constantly engaged in the pursuit and capture of their prey, the degree of their power in these respects depending on the dimensions of the mouth and gullet and the strength of the teeth and jaws. If the teeth are sharp and hooked, they are capable of securing the most slender and agile animals; if this kind of teeth is combined with a wide gullet and distensible stomach, they are able to overpower and swallow other fish larger than themselves; if the teeth are broad, strong molars, they are able to crush the hardest aliments; if they are feeble, they are only serviceable in procuring some small or inert and unresisting prey. Teeth may be wanting altogether. Whatever the prey, in the majority of cases it is swallowed whole; but some of the most voracious fishes, like some Sharks and Characinidæ, are provided with cutting teeth, which enable them to tear their prey to pieces if too large to be swallowed whole. Auxiliary organs for the purpose of overpowering their prey, which afterwards is seized or torn by the teeth, like the claws of some carnivorous mammals and birds, are not found in this class; but in a few fishes the jaws themselves are modified for that purpose. In the Sword-fishes the bones of the upper jaw form a long dagger-shaped weapon, with which they not only attack large animals, but also frequently kill fishes on which they feed. The Saw-fishes are armed with a similar but still more complicated weapon, the saw, which is armed on each side with large teeth implanted in deep sockets, specially adapted for killing and tearing the prey before it is seized and masticated by the small teeth within the mouth. Fishes show but little choice in the selection of their food, and some devour their own offspring indiscriminately with other fishes. Their digestive powers are strong and rapid, but subject in some degree to the temperature, which, when sinking below a certain point, lowers the vital powers of these cold-blooded animals. On the whole, marine fishes are more voracious than those inhabiting fresh waters; and whilst the latter may survive total abstinence from food for weeks or months, the marine species succumb to hunger within a few days. The growth of fishes depends greatly on the nature and supply of food, and different individuals of the same species may exhibit a great disparity in their respective dimensions. They grow less rapidly and to smaller dimensions in small ponds or shallow streams than in large lakes and deep rivers. The young of coast fishes, when driven out to sea, where they find a much smaller supply of food, remain in an undeveloped condition, assuming an hydropic appearance. The growth itself seems to continue in most fishes for a great length of time, and we can scarcely set bounds to--certainly we know not with precision--the utmost range of the specific size of fishes. Even among species in no way remarkable for their dimensions we sometimes meet with old individuals, favourably situated, which more or less exceed the ordinary weight and measurement of their kind. However, there are certain evidently short-lived species of fishes which attain a remarkably uniform size within a very short time; for instance, the Stickleback, many species of Gobius and Clupea.

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The organs of nutrition, manducation, and deglutition, are lodged in two large cavities--an anterior (the mouth or buccal cavity), and a posterior (the abdominal cavity). In the former the alimentary organs are associated with those fulfilling the respiratory functions, the transmission of food to the stomach and of water to the gills being performed by similar acts of deglutition. The abdominal cavity commences immediately behind the head, so, however, that an extremely short thoracic cavity for the heart is partitioned off in front. Beside the alimentary organs it contains also those of the urogenital system and the air-bladder. The abdominal cavity is generally situated in the trunk only, but in numerous fishes it extends into the tail, being continued for some distance along each side of the hæmal apophyses.

In numerous fishes the abdominal cavity opens outwards by one or two openings. A single porus abdominalis in front of the vent is found in Lepidosiren and some Sturgeons; a paired one, one on each side of the vent, in Ceratodus, some species of Sturgeon, Lepidosteus, Polypterus, Amia, and all Chondropterygians. As in these fishes semen and ova are discharged by proper ducts, the abdominal openings may serve for the expulsion of semen, and those ova only which, having lost their way to the abdominal aperture of the oviduct, would be retained in the abdominal cavity. In those Teleosteans which lack an oviduct a single porus genitalis opens behind the vent.

The mouth of fishes shows extreme variation with regard to form, extent, and position. Generally opening in front, it may be turned upwards, or may lie at the lower side of the snout, as in most Chondropterygians, Sturgeons, and some Teleosteans. Vogt regards this position as a persistent fœtal condition. In most fishes the jaws are covered by the skin, which, before passing over the jaws, is often folded, forming more or less fleshy lips. In the Sharks the skin retains its external character even within the teeth, but in other fishes it changes into a mucous membrane. A tongue may exist as a more or less free and short projection, formed by the glosso*-hyal and a soft covering, or may be entirely absent. Salivary glands and a velum palati are absent in fishes.

With regard to the dentition, the class of Fishes offers an amount of variation such as is not found in any of the other classes of Vertebrates. As the teeth form one of the most important elements in the classification of fishes, their special arrangement and form will be referred to in the account of the various families and genera. Whilst not a few fishes are entirely edentulous, in others most of the bones of the buccal cavity, or some of them, may be toothed, as the bones of the jaws, the palatines, pterygoids, vomers, basisphenoid, glossohyal, branchial arches, upper and lower pharyngeals. In others teeth may be found fixed in some portion of the buccal membrane without being supported by underlying bone or cartilage; or the teeth have been developed in membrane overlying one of the dentigerous bones mentioned, without having become anchylosed to the bone. When the tooth is fixed to the bone the attachment has generally been effected by the ossification of the bone of the tooth, but in some fishes a process of the bone projects into the cavity of the tooth; in others the teeth are implanted in alveoli. In these, again, frequently a process of bone rises from the bottom, on which the tooth rests.

Many fishes, especially predatory fishes with long, lancet-shaped teeth, have all or some of the teeth capable of being bent inwards towards the mouth. Such “hinged” teeth resume at once the upright position when pressure is removed from them. They are, however, depressible in one direction only, thus offering no obstacle to the ingress, but opposing the egress of prey. Mr. C. S. Tomes has shown that the means by which this mechanism is worked are different in different fishes; for whilst, in the Pediculati and Gadoids (Hake) the elasticity resides solely in the tissue of the hinge (the tooth being as resilient as ever after everything else is severed), in the Pike the hinge is not in the least endowed with elasticity, but the bundles of fibres proceeding from the interior of the dentine cap are exceedingly elastic.

As regards texture the teeth of fishes show considerable variation. The conical teeth of the Cyclostomes and the setiform teeth of many Teleosteans consist of a horny albuminous substance. The principal substance of the teeth of other fishes consists of dentine, with numerous dividing and anastomosing tubercles, sometimes covered by a stratum of unvascular dentine. An enamel-like substance has been observed on the crown of the teeth of Sargus and Balistes, and an ossification of the capsule of their matrix covers the enamel with a thin coating of cement. The teeth either possess a cavity in which the matrix is received, or, more frequently, they are solid, in which case vascular canals of the underlying bone are continued into the substance of the tooth. In the teeth of some fishes numerous sets of canals and tubes are so arranged that they do not anastomose with one another, each set being surrounded by a layer of dentine and cement. These apparently simple teeth are evidently composed of numerous small teeth, and called compound teeth.

The teeth may be, and generally are, very different as regards size or form in the different parts of the mouth; they may be also different according to the age or sex of the fish (Raja). The teeth may be few in number and isolated, or placed in a single, double, or triple series, distant from one another or closely set; they may form narrow or broad bands, or patches of various forms. As regards form, they may be cylindrical or conical, pointed, straight, or curved, with or without an angular bent near their base; some are compressed laterally or from the front backwards; the latter may be triangular in shape, or truncated at the top like incisors of mammals; they may have one apex (cusp) only, or be bi- or tri-lobate (bi- or tri-cuspid); or have the margins denticulated or serrated. Compressed teeth may be confluent, and form a cutting edge in both jaws, which assume the shape of a parrot’s beak (Fig. 53). In some the apex is hooked or provided with barbs. Again, some teeth are broad, with flat or convex surface, like molar teeth. With regard to size, the finest teeth are like fine flexible bristles, ciliiform or setiform; or, if very short and anchylosed to the bone, they appear only as inconspicuous asperities of the bone. Very fine conical teeth arranged in a band are termed villiform teeth; when they are coarser, or mixed with coarser teeth, they are card-like (dents en rape or en cardes) (Fig. 54); molar-like teeth of very small size are termed granular.

In all fishes the teeth are constantly shed or renewed during the whole course of their life. In fishes which have compound teeth, as the Dipnoi, Chimæroids, Scari, Gymnodonts, as well as in those which have apparently permanent teeth, as in the saw of Pristis, the detrition of the surface is made up by a constant growth of the tooth from its base. When the teeth are implanted in alveoli, they are generally succeeded by others in the vertical direction, but in others they succeed one another, side by side. In the majority of fishes the new tooth is not developed (as in reptiles and mammals) in a diverticulum of the sack of its predecessor, but like this from the free surface of the buccal membrane. Generally there are more than one tooth growing, which are in various stages of development, and destined to replace the one in function. This is very conspicuous in Sharks, in which the whole phalanx of their numerous teeth is ever marching slowly forwards (or in some backwards), in rotatory progress, over the alveolar border of the jaw, the teeth being successively cast off after having reached the outer margin, and fulfilled for a longer and shorter period their special function.

The intestinal tract is divided into four portions: œsophagus, stomach, small and large intestine; two or more of these divisions may coalesce in fishes and become indistinguishable. But it is characteristic of the class that the urinary apertures are constantly situated behind the termination of the intestinal tract.

In Branchiostoma the whole intestinal tract is straight, and coated with a ciliated mucous membrane. The wide pharynx passes into a narrow œsophagus, this into a gastric cavity, the remainder being again narrower and terminating in the anal aperture, which lies somewhat to the left of the median line. The liver is represented by a green coloured cœcal diverticulum of the stomachic dilatation. A mesenterium is absent.

In the Cyclostomi the intestinal tract is likewise straight, and without clearly defined divisions; however, in Petromyzon the œsophagus shows numerous longitudinal folds, and the intestine proper is provided with a single longitudinal fold. A mesentery, which is present in the Myxinoids, is represented by a short median fold only, by means of which the hindmost part of the intestine is fixed.

The Palæichthyes show differences in the structure of their intestinal tract as considerable as are found among the Teleostei, but they have that in common that the absorbent surface of their intestine is enlarged by the development of a spiral valve, evidence of the presence of which in extinct Palæichthyes is still preserved in the fossilised fæces or coproliths, so abundant in some of the older strata.

In Chondropterygians (Fig. 55) the stomach is divided into a cardiac and pyloric portion, the former frequently terminating in a blind sac, and the latter varying in length. The pyloric portion is bent at its origin and end, and separated from the short duodenum (called Bursa entiana in these fishes) by a valve; the ductus hepaticus and pancreaticus enter the duodenum. This is succeeded by the straight intestine provided with the spiral valve, the coils of which may be either longitudinal and wound vertically about the axis of the intestine, as in Carcharias, Galeocerdo, Thalassorhinus, and Zygœna, or they may be transverse to that axis, as in the other genera. The number of gyrations in the latter case varies: there may be as many as forty. The short rectum passes into a cloaca, which contains also the orifices of the urogenital ducts. Only the commencement and end of the intestinal tract are fixed by mesenterial folds.

In the Holocephali and Dipnoi, the intestinal tract is short, straight, and wide, without stomachic dilatation, a pyloric valve, close to which the ductus choledochus enters, indicating the boundary of the intestine proper (Fig. 57, p). The spiral valve is perfect, and makes from three (Chimæra) to nine (Ceratodus) gyrations. A cloaca is present, as in Chondropterygians. A mesentery fixing the dorsal side of the intestine is absent.

a, Œsophagus; b, Cardiac portion of stomach; c, pyloric portion; d, pouch intermediate between stomach and duodenum, with circular valves at both ends; e, Duodenum; f, Valve of intestine; g, Ductus hepaticus; h, Spleen.]

The other Ganoids resemble again more the Chondropterygians in the structure of their intestinal tract. The stomach has always a distinct pyloric portion, and has a still more complicated structure in Acipenser. The duodenal portion receives the contents of Appendices pyloricæ, which are confluent into a gland-like mass in Acipenser, but separate in Polyodon, and numerous and short in Lepidosteus, whilst Polypterus possesses one such appendage only. A spiral valve is developed in the Sturgeons and Polypterus, but in Amia, in which the intestine performs several convolutions, the four gyrations of the valve are situated far back towards the end of the intestine. In Lepidosteus the valve is rudimentary, and indicated only by three raised lines crossing the terminal portion of the intestine. In all these Ganoids the rectum has a separate opening, without cloaca.

The structure of the intestinal tract of Teleosteous fishes is subject to so numerous modifications that we should go beyond the limits of the present work if we would attempt to enter into details. Great differences in this respect may be found even in groups of the same natural families. Frequently the intestinal tract remains of nearly the same width throughout its course, and only the entrance of the various ducts serves as a guide for the distinction of its divisions. An intestine of such uniform width may be straight and short, as in Scombresocidæ, Symbranchidæ, or it may be more or less convoluted and long, as in many Cyprinidæ, Doradina, etc. On the whole, carnivorous fishes have a much shorter and simpler intestinal tract than herbivorous.

In the majority of Teleosteans, however, œsophagus, stomach, duodenum, small intestine and rectum, can be more or less distinctly, even externally distinguished.

There are two predominant forms of the stomach, intermediate forms being, however, numerous. In the first, the siphonal, it presents the form of a bent tube or canal, one-half of the horse-shoe being the cardiac, the other the pyloric portion. In the second, the cæcal, the cardiac division is prolonged into a long descending blind sac, the cardiac and pyloric openings of the stomach lying close together (Clupea, Scomber, Thynnus, etc.)

The duodenum receives always the hepatic and pancreatic secretions, and, besides, those of the appendices pyloricæ, which, in varying numbers (from 1 to 200), are of very common occurrence in Teleosteans (Fig. 56). They vary also in length and width, and whilst the narrowest serve only as secretory organs, the widest are frequently found filled with the same contents as the intestine. When few in number, each opens by a separate duct into the duodenum; when their number is greater two or more coalesce into a common duct; in the latter case the appendages cease to be free, and are connected with one another by a more or less firm tissue.

Cœcal appendages at the end of the intestinal tract are of exceedingly rare occurrence in fishes (Box). There is no cloaca in Teleosteans.

In the majority of Teleosteous fishes the vent is situated on the boundary between trunk and tail, behind the ventral fins. In a few it lies farther backwards, not far from the caudal fin; more frequently it is advanced forwards, under the middle of the abdomen or to the scapular arch. In two fishes, Aphredoderus and Amblyopsis, it lies before the pectoral fins.

A peritoneum envelops all the divisions of the intestinal tract within the abdominal cavity. A broad, well-developed omentum has hitherto been found in Gobiesox cephalus only.

Liver.--The existence of a liver in Branchiostoma as a long diverticulum of the intestine has been mentioned above. In the Myxinoids the liver is divided into two glandular bodies, an anterior rounded smaller one, and a posterior larger one of an elongate shape. The gall-bladder lies between both, and receives a cystic duct from each of them. In the other fishes the proportionally large liver is a single large gland, from which only now and then small portions are found to be detached. It is either simple, or with a right and left lobe, or with a third lobe in the middle; each lobe may have incisions or subdivisions, which, however, are very inconstant. The liver of fishes is distinguished by the great quantity of fluid fat (oil) which it contains. The gall-bladder is but rarely absent, and attached to the right lobe, or towards the centre; however, in some fishes it is detached from the liver and connected with it by the cystic duct only. The bile may be conveyed by one or more hepatic ducts into a common duct which is continued towards the gall-bladder as ductus cysticus, and towards the duodenum as ductus choledochus; or some of the hepatic ducts enter directly the gall-bladder, or directly the duodenum, without communicating with the common duct. Individual variations in this respect are of common occurrence.

A pancreas has been found hitherto in all Chondropterygians, Acipenser, and many Teleosteans. In the first it is a glandular mass of considerable size behind the stomach, close to the spleen; its duct leads into the duodenum. In the Sturgeons the pancreas is attached to the duodenum, and opens close to the ductus choledochus. In Silurus glanis it is very large, and the ductus choledochus passes through its substance; it is smaller in Belone and Pleuronectes, and situated in the mesentery; its duct accompanies the terminal portion of the ductus choledochus. In the Salmon, which possesses a large lobed pancreas, the duct is so intimately connected with the ductus choledochus that both appear externally as a single duct only.

The spleen, which is substantially a lymphatic gland, may be mentioned here, as it is constantly situated in the immediate vicinity of the stomach, generally near its cardiac portion. With the exception of Branchiostoma, it is found in all fishes, and appears as a rounded or oblong organ of dark-red colour. In the Sharks frequently one or more smaller pieces are detached from the principal body. In the Dipnoi a thin layer of a very soft substance of brownish-black colour below the mucous membrane of the stomach and upper part of the intestine has been regarded as the homologue of the spleen (Fig. 57, m). In most Teleostei the spleen is undivided, and appended by its vessels and a fold of the peritoneum to the pyloric bend of the stomach or the beginning of the intestine.

é, Mouth of ductus choledochus; f, stomach; i, adipose agglomeration; l, first compartment of intestinal spire; m, spleen; oe, lower part of œsophagus, opened; p, double pyloric fold; q q, glandular patches.]