F. ORGANS OF NUTRITION. 



ALIMENTARY CANAL AND ITS APPENDAGES. 



The alimentary canal consists of a tube which begins at the 

 aperture of the mouth, passes through the body cavity (ccelome), 

 and ends at the vent or anus}- Its walls consist of several layers 

 (Fig 214,), of which the mucous membrane, lining the cavity of 

 the tube, and the muscular layer external to this, extend throughout 

 the canal. The mucous membrane consists of a superficial 

 epithelium and a deeper connective-tissue layer, the outer part 

 of which, or sub-mucosa, forms a loose network extending to the 

 muscular layer. The epithelium is derived from the hypoblast, 

 with the exception of that lining the mouth and anus (stomodwum 

 and proctodceum*) which is epiblastic in origin (p. 5). The con- 

 nective tissue and muscular layers arise from the splanchnic layer 

 of mesoblast of the embryo ; and the muscular coat, consisting 

 almost entirely of unstriated fibres, supplied with nerves from the 

 sympathetic system, is, as a rule, divided into two layers, the inner 

 being constituted by circular, and the outer by longitudinal fibres. 

 These serve for the contraction or peristalsis of the wall of the gut, 

 and thus fulfil the double function of bringing the nutritive con- 

 tents of the latter into the closest possible relation with the whole 

 epithelial surface, and at the same time of removing from the body 

 the substances which have not been absorbed. Striated (voluntary) 

 muscular fibres, supplied by cerebral or spinal nerves, occur only at 

 the anterior and posterior ends of the canal. 



An outer accessory serous coat, the peritoneum, encloses the 

 gut externally in the region of the ccelome. This is covered on its 



1 In embryos of many Vertebrates (e.g., Elasmobranchii, Amphibia), a pig- 

 mented ridge of cells is formed on the dorsal side of the gilt in the head and trunk, 

 and gives rise to a rod lying close beneath the notochord. In certain cases it 

 remains for a time in connection with the gut by a series of segmental canals 

 which later disappear. The meaning and subsequent fate of this aub-notochordod 

 rod or hypochorda are not known. 



2 Phylogenetically the proctodaeum is older than the stomodaeum, and in many 

 Vertebrates it is derived directly from the blastopore. 



