A protrusible tongue is never developed in Fishes. A rudiment of that organ is present in the Elasmobranchs (Fig. 153) and Dipnoi, and also in the Crossopterygii, and usually consists of an elevated area of mucous membrane provided with free lateral edges and a forwardly projecting apex; it is supported by the basi-hyal element of the hyoid arch. In the Crossopterygii (e.g. Polypterus) the tongue contains muscle fibres, and in the Dipnoi, where the organ is better developed than in any other Fishes, special lingual muscles are present.
The pharynx succeeds the oral cavity, and is perforated on each side by the branchial clefts (Figs. 153, 154). The rest of the alimentary canal differs considerably in various Fishes in the degree of distinctness of its several regions, and in the extent to which it is convoluted. As a rule the pharynx is followed in succession by an oesophagus, a stomach, and an intestine (Fig. 153), the latter terminating in a portion usually termed the "rectum." The boundaries of these regions are not always very obvious, but are indicated by variations in calibre, by changes in the character of the lining epithelium, by special valves or sphincter muscles, or by the entrance of the ducts of certain glands like the pancreas and liver.
Fig. 153.—Dissection of a male Dog-Fish (Scyllium). The left side of the body is cut away to the median plane so as to expose the abdominal and pericardial cavities and the neural canal in their whole length. The alimentary canal and the liver have been drawn downwards, and the oral cavity, the pharynx, part of the intestine, and the cloaca have been opened. The cartilaginous parts of the skeleton are dotted, and the calcified portions of the vertebral centra are black. abd.cav, Abdominal cavity; au, auricle; b.br, basi-branchial; b.hy, basi-hyal; c.art, conus arteriosus; cd.a, caudal artery; cd.st, cardiac part of the stomach; cd.v, caudal vein; cl, cloaca; cn, centrum; cr, cranium; crb, cerebellum; d.ao, dorsal aorta; dien, thalamencephalon; epid, epididymis; fon, fontanelle; gul, oesophagus; h.a, haemal arch; i.br.a1-i.br.a5, internal gill-clefts; int, intestine; kd, kidney; l.j, lower jaw; l.lr, left lobe of liver; med.obl, medulla oblongata; mes, mesentery; n.a, neural arch; n.cav, neural canal; olf.l, olfactory lobes; opt.l, optic lobes; pan, pancreas; pcd.cav, pericardial cavity; pct.a, pectoral arch; ph, pharynx; pin, pineal body; p.n.d, vestigial Müllerian duct; prs, prosencephalon; pty, pituitary body; pv.a, pelvic arch; pyl.st, pyloric portion of the stomach; r, rostrum; r.lr, right lobe of liver; rct.gl, rectal gland; sp, spiracle; sp.cd, spinal cord; spl, spleen; sp.s, sperm sac; sp.vl, spiral valve; s.v, sinus venosus; tng, tongue; ts, testis; u.g.s, urino-genital sinus; u.j, upper jaw; ur, metanephric duct; v, ventricle; v.ao, ventral aorta; v.def, vas deferens or mesonephric duct; vs.sem, vesicula seminalis. (From Wiedersheim, after T. J. Parker.)
The oesophagus is occasionally separated from the stomach by a slight constriction, but more frequently the replacement of the squamous epithelium of the oesophagus by the columnar epithelium of the stomach and the appearance of gastric glands in the wall of the latter cavity afford the only distinction between the two regions. The commencement of the intestine is usually indicated by a pyloric "valve" (Fig. 155, A, B), in the form of a ring-like, inwardly projecting thickening of the circularly-disposed muscle fibres of the terminal extremity of the stomach, and usually also by the entrance of the distinct or united ducts of the liver and pancreas; sometimes, as in certain Elasmobranchs and in the Dipnoi, by a special dilatation or "Bursa Entiana" (Fig. 155, A). The rectum, or terminal portion of the intestine, is distinguished from the rest of the gut by its straight course to the cloacal aperture or the anus, and sometimes by an increase in calibre. In Box vulgaris and a few other Teleosts[[226]] a caecal diverticulum indicates the commencement of the rectum, while in a few cases the pre-rectal portion of the intestine communicates with the enlarged rectal segment by a much constricted valvular orifice which is suggestive of the ileo-colic valve of the higher Vertebrates,[[227]] as in the Teleosts Amiurus catus,[[228]] Trigla gurnardus, and Cyclopterus lumpus.
The relation of the regional divisions of the intestine in Fishes to those of other Vertebrates are somewhat difficult to determine. If we may regard the "rectal" gland of Elasmobranchs and the intestinal caecum of certain Teleosts as homologous with each other, and with the caecum coli of the higher Vertebrates, then it would seem that by far the greater part of the intestine of Fishes, including that portion in which a spiral valve may be developed, is homologous with the pre-caecal segment of the gut or small intestine in other Vertebrates, and that the post-caecal section, or large intestine, of the latter is represented in Fishes only by that relatively short portion of the gut which lies posterior to the rectal gland or its homologue in Teleosts, the equivalent of the colon of Mammalia being, as in Amphibia, Reptiles, and Birds, practically undifferentiated.[[229]]
In the Cyclostomata the alimentary canal retains much of its primitive simplicity. It pursues a straight course from mouth to anus, and the usual regions are very obscurely indicated. The same remarks apply also to the Holocephali and a few Teleosts, although in these Fishes the limits of the different regions are somewhat more clearly defined. In the Dipnoi (Fig. 155, A), a contracted sigmoid curve between the somewhat dilated stomach and the spacious intestine is the only departure from the straight course of the preceding groups.
Fig. 154.—Dissection of a male Teleost (Salmo fario) from the left side. a.bl, Air-bladder opened; an, anus; au, auricle; b.a, bulbus aortae; B.HY, basi-hyal; B.OC, basioccipital; cd.a, caudal artery; cd.v, caudal vein; CN, centrum; crb, cerebellum; d.f.1, first dorsal fin; D.F.R, dermal fin-rays; du, duodenum or anterior segment of the intestine; FR, frontal; g.bl, gall-bladder; gul, oesophagus or gullet; H.SP, haemal spine; int, intestine; kd, kidney; kd′, "head-kidney"; lg, tongue; lr, liver; N.SP, neural spine; opt.l, optic lobes; PA.SPH, parasphenoid; ph, pharynx; pn.b, pineal body; pn.d, bristle passed into ductus pneumaticus; prsen, prosencephalon; pty.b, pituitary body; PTG, pterygiophores, or radial elements of dorsal and ventral fins; pv.f, pelvic fin; py.c, pyloric caeca; S.ETH, supra-ethmoid; S.OC, supra-occipital; spl, spleen; st, stomach; ts, testis; u.bl, urinary bladder; u.g.s, urino-genital sinus and its external aperture; ur, ureter or kidney-duct; v, ventricle; v.ao, ventral aorta; v.df, vas deferens; v.f, ventral fin; VO, vomer. (From Parker and Haswell.)
In the remaining Fishes the degree of convolution varies within rather wide limits. The oesophagus is usually straight and wide, but in Lutodeira, among Teleosts, it is long and even convoluted, and in the Plectognath Teleosts it gives off a large sac-like outgrowth ("air-sac"), which extends anteriorly as far as the head, and posteriorly to the beginning of the tail, and communicates with the oesophagus by two apertures. The stomach may be U-shaped with the concavity directed forwards, and consisting of a right limb passing backwards from the oesophagus, and a left limb curving forwards to its junction with the intestine (Fig. 153). In such instances as these the stomach and the adjacent section of the intestine describe a characteristic siphonal curve. In certain other Fishes (Fig. 160), the oesophageal portion of the stomach terminates behind in a tubular or sac-like dilatation at some distance posterior to the laterally situated pylorus, which indicates the origin of the intestine. The intestine is straight, or nearly so, in Elasmobranchs, Crossopterygii, and Dipnoi, and also in a few Teleosts; but sometimes, and very generally in Teleosts, it is more or less convoluted, notably in some of the Mugilidae, and in the Loricariidae, where, as in Plecostomus, it is disposed in numerous spiral coils like a watch-spring. The terminal portion of the intestine or rectum either opens into a cloaca, which also receives the urinary and genital ducts, as in Elasmobranchs (Fig. 153), and Dipnoi (Fig. 155, A), or opens externally by an anus, situated in front of the separate or united urinogenital ducts, as is the case with all the remaining groups of Fishes (Fig. 154). The cloacal aperture is invariably situated near the junction of the caudal and trunk regions, and as a rule is median in position, rarely, as in the Dipnoi, displaced to the right or left of the middle line; but the anus differs greatly in position, sometimes retaining its primitive position at the hinder end of the trunk, as in the Holocephali, Chondrostei, Crossopterygii, Holostei, and many Teleosts, or occupying almost any position between that point and, as in the "Electric Eels" (Gymnotidae), the ventral surface of the throat (Fig. 351.)