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 {259}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] [Illustration: 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 {260}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]