Fig. 155.—A, alimentary canal and liver of a female Protopterus, from the left side. Part of the left wall of the stomach and intestine, and the peritoneal investment of the spleen have been removed. a.p, Abdominal pore; b.d, bile-duct; b.ent, Bursa Entiana; cl, cloaca; cl.ap, cloacal aperture; cl.c, caecum cloacae; c.m.a, coeliaco-mesenteric artery; cy.d, bile duct; k.d, kidney duct; m.a, mesenteric arteries; od, oviduct; pt.c, post-caval vein or inferior vena cava; p.v, portal vein; the other reference letters as in B. (From Newton Parker.) B, viscera of an adult female Lepidosteus, ventral view. The oesophagus, the commencement of the intestine and the rectum have been laid open. ab, air-bladder; an, anus; b.d, intestinal aperture of the bile-duct; g.b, gall-bladder; gl, oesophageal aperture of the air-bladder; h.d, hepatic duct; l, liver; oes, oesophagus; py, pylorus; py.c; pyloric caeca; py.c′, the four intestinal orifices of the pyloric caeca; r, rectum; s, spleen; sp.v, spiral valve; st, stomach. (From Balfour and Newton Parker.)

Fig. 156.—Transverse section of a Fish, diagrammatic. cn, Centrum; coel, coelome; d.a, dorsal aorta; d.f, dorsal fin; d.m, dorsal muscles; d.ms, dorsal mesentery; f.r, fin ray; gon, gonad; int, intestine; l.v, lateral vein; msn, mesonephros; msn.d, mesonephric duct; n.a, neural arch; p, parietal layer of the peritoneum; p′, visceral layer; p.c.v, posterior cardinal vein; pn.d, Müllerian duct; r, ventral rib; r′, dorsal rib; sp.c, spinal cord; t.p, transverse process; v.m, ventral muscles; v.ms, ventral mesentery. (Modified, after Parker and Haswell.)

The whole length of the alimentary canal from the oesophagus to the rectum is invested externally by the visceral layer of the peritoneum (Fig. 156), which histologically consists of a stratum of connective tissue, supporting on its free surface an epithelial stratum (coelomic epithelium). Primarily, the investing peritoneum is continued both dorsally and ventrally into bilaminar suspensory folds, the dorsal and ventral mesenteries (d.ms, v.ms), which extend to the mid-dorsal or mid-ventral line of the abdominal cavity. The two layers then separate and become continuous with the parietal layer of the peritoneum lining the whole of the inner surface of the body-wall. Embryologically, the two mesenteries owe their formation to the fusion above and below the mesenteron of the contiguous walls of two laterally situated and primitively distinct coelomic cavities. The dorsal mesentery in the adult is occasionally complete, as in the Myxinoid Cyclostomata and in the Elasmobranch Hypnos subnigrum,[[230]] and also in some Dipnoi and in a few Teleosts, but much more frequently it is reduced by absorption to anterior and posterior remnants, or to a series of isolated bands, or even, as in the Lamprey (Petromyzon), to a few filaments accompanying the intestinal blood-vessels. The ventral mesentery, on the contrary, is rarely present, and if present is never complete. In Lepidosteus[[231]] a ventral mesentery is said to be present in connexion with that part of the intestine which contains the spiral valve. In Protopterus,[[232]] and also in Neoceratodus,[[233]] there is a well-developed ventral mesentery in relation with the greater part of the length of the intestine, although in the former Dipnoid its continuity is interrupted by one or two vacuities, and in the latter the mesentery is incomplete posteriorly. A ventral mesentery is also present in the intestinal region of some of the Muraenidae among Teleosts.[[234]]

Fig. 157.—Transverse section through a portion of the wall of the intestine, combined from the condition seen in both the higher and the lower Vertebrata. Semi-diagrammatic. a.c, Epithelial cells in the amoeboid state; b.v, blood-vessels; c.m, circular muscular layer; g, one of Lieberkühn's glands in the higher Vertebrates; i.ep, intestinal epithelium; l, leucocytes; l′, leucocytes in the intestinal epithelium; l.f, lymph follicles; l.m, longitudinal muscular layer; lym, lymphatic vessels; p, visceral layer of the peritoneum; sm, the submucosa; v, villi of the higher Vertebrates. (From Wiedersheim.)

Internal to its peritoneal investment the wall of the alimentary canal consists in succession from without inwards of (1), a muscular coat, (2) the submucosa, and (3) an epithelial stratum or mucous membrane, the first two of these layers, with the addition of the peritoneum, being derivatives of the inner or splanchnic portion of the embryonic mesoblast.[[235]]

Excluding the oesophagus, where the muscular coat is mainly composed of striated fibres, the musculature of the alimentary canal usually consists solely of non-striated, spindle-shaped fibres disposed in two layers, an external stratum of longitudinally arranged fibres, and an inner stratum of circularly disposed fibres (Fig. 157), with the addition, in the stomach, of an oblique layer between the two. In the oesophagus the reverse arrangement may exist, the circular layer being external and the longitudinal internal. The muscular coat varies considerably in thickness in different regions and in different Fishes, and in the Cyclostomata, the Holocephali, some Teleosts, and the Dipnoi may be very feebly developed, or even entirely absent, as in the intestine of the Hag-Fish (Myxine). In the Gillaroo Trout (Salmo stomachicus),[[236]] on the contrary, the distal section of the siphonal stomach has its musculature unusually thickened, so as to form an incipient gizzard for the crushing of the shells of the freshwater Molluscs on which the Fish feeds. In some of the Mullets (Mugilidae),[[237]] a true gizzard is developed by the enormous thickening of the muscular coat of the caecal stomach, the cavity of which, in consequence, is reduced to a mere vertical fissure, and is lined by an exceptionally thick, horny epithelium.

There are a few exceptions to the rule that the muscular fibres are of the non-striated variety. Thus in some Teleosts, as in the Tench (Tinca vulgaris), striated fibres are continued from the oesophagus into the walls of the stomach and intestine, and there form an outer longitudinal and an inner circular layer, situated externally to the corresponding layers of the non-striated stratum. In the Siluroid, Amiurus, the striated fibres of the outer circular layer of the oesophagus are continued, although but sparsely, into the inner circular layer of the stomach.