DIGESTIVE APPAEATUS. 



483 



Fig. 244. 



to them in regular and quick succession, which produces the appearance 

 of waves ; and each wave answers here to a tooth. 



(1267.) Whatever little substances, alive or inanimate, the current 

 of water brings into the branchial sac, if not rejected as unsuitable, 

 lodge somewhere on the respiratory surface, along which each particle 

 travels horizontally, with a steady, slow 

 course to the front of the cavity, where 

 it reaches a downward stream of similar 

 materials ; and they proceed together, 

 receiving accessions from both sides, and 

 enter at last the oesophagus, placed at the 

 bottom (fig. 244, #), which carries them, 

 without any effort of swallowing, towards 

 the stomach. 



(1268.) The oesophagus (fig. 244, h) 

 is short, and internally gathered into 

 longitudinal folds. The stomach (i) is 

 simple, moderately dilated, and has its 

 walls perforated by several orifices, 

 through which the biliary secretion en- 

 ters its cavity. The liver is a glandular 

 mass intimately adherent to the exterior 

 of the stomach and the intestinal canal 

 (fig. 243, e e), of variable length, and, 

 more or less convoluted in different spe- 

 cies, after one or two folds terminates in 

 the rectum, which, emerging from the 

 peritoneal investment covering the intes- 

 tine, has its extremity loosely floating in 

 the cavity communicating with the second 

 orifice (/) : into the latter a bristle is 

 introduced in the figure, having its ex- 

 tremity inserted into the anal extremity 

 of the digestive tube. Excrementitious 

 matter, therefore, when discharged from 

 the rectum, escapes from the body through 



the common excretory aperture, generally situated upon the least ele- 

 vated protuberance of the outer covering*. It would seem that the 

 food of Ascidians consists of very minute particles of organized matter ; 

 for although small Crustacea and other animal remains have been occa- 

 sionally met with in the branchial chamber, nothing of this nature has 

 been observed in the stomach itself ; and, as must be obvious to the 

 reader, the oral aperture seems but little adapted to the deglutition of 

 bulky substances. 



* Cuvier, Memoire sur les Ascidies, p. 14. 



2i 2 



Diagram of an Ascidian, showing the 

 position of the viscera: a, the oral 

 orifice ; b, tentacula guarding it ; c, d, 



mach and intestine ; o, ovary ; p, termi- 

 nation of oviduct ; q, common excretory 

 orifice. 



