F. ORGANS OF NUTRITION. 



ALIMENTARY CANAL AND ITS APPENDAGES. 



The alimentary or enteric canal consists of a tube which 

 begins at the aperture of the mouth, passes through the body- 

 cavity (coelome), and ends at the vent or anus. 1 The walls of 

 the canal consist of several layers, of which the mucous mem- 

 brane, lining the cavity, 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 submucosa, forms a loose network con- 

 necting it with the muscular layer.' 2 The epithelium is derived 

 from the endoderm, with the exception of that lining the mouth 

 and anus (stomodccum and proctodoeum) which is ectodermic in origin 

 (p. 5). The connective tissue and muscular layers arise from the 

 splanchnic layer of mesoderm of the embryo ; and the muscular 

 coat, consisting almost entirely of unstriated fibres, supplied by 

 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 contents 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, only occur 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 coelome. This is covered on its 

 free surface by pavement epithelium, and, dorsally to the alimentary 

 canal, is reflected round the entire body-cavity, converting the 



1 The mouth of Amphioxus apparently corresponds to the first gill-cleft of the 

 left side of craniate embryos (i.e. to the left spiracle of Fishes). The mouth of 

 Craniates is probably a new acquisition (neoxtoma, cf. p. 203), which has arisen 

 by the confluence of'a pair of gill-clefts. The anus, which in many Vertebrates 

 arises directly from the blastopore, is phylogenetically older than the neostoma. 



2 A layer of smooth muscular fibres may be present in the submucosa, which 

 also encloses lymphoid or adenoid tissue (solitary follicles, Peyer's jMtches). 



