362 ORGANS OF DIGESTION. 



stomach, and also a part of the intestine, are generally sur- 

 rounded with the soft granular substance of the liver, which 

 presents an appearance of minute lobules from the grouping 

 of its component follicles. The intestine, on leaving the 

 stomach, forms a sigmoid curvature in the abdominal cavity 

 on the back part of the respiratory sac, ascending towards the 

 exterior vent near which it generally terminates with a 

 fringed margin (88. .). The intestine, covered with perito- 

 neum, highly vascular, without mesentery, and without coecal 

 or other enlargements, occupies a cavity not traversed by the 

 respiratory currents, and has on its convex posterior part, the 

 heart and aorta, and on its upper or anterior part, the ovaria 

 (88. k.) and oviducts (88. m.) 9 as in the bivalved mollusca. 

 The respiratory currents, after traversing the ciliated perfora- 

 tions of the branchiae, pass out by a distinct canal, over the 

 anal aperture and the generative orifices, to the exterior vent, 

 so that the ex-currents, aided by the contractions of the 

 general muscular tunic, assist in expelling the products of 

 generation and the residue of digestion, as in the conchifera. 

 The respiratory orifice, for the entrance of the currents and 

 of the food, is generally larger than the vent by which they 

 are expelled, and the two apertures of the alimentary canal, 

 the mouth and anus, are variously approximated to the 

 exterior orifices of the enveloping tunic in the different species 

 of this class. The anal aperture of the intestine, which pro- 

 jects free into the expiratory canal, is generally lobed or 

 fimbriated or valvular as in many of the higher mollusca. In 

 some of the compound tunicata the liver is not distinguish- 

 able ; in the pyrosoma it is divided longitudinally into several 

 lobes which communicate with the intestine by distinct ducts ; 

 in some cynthiae it forms a glandular layer of minute follicles 

 over a part of the intestine, or of the stomach, as in the 

 cynthia canopus, and in others, as the cynthia momus, the 

 liver presents a more definite character and form, as in higher 

 classes. The convoluted alimentary canal is commonly filled 

 with a dark-coloured flocculent mucous matter, like that 

 found in the intestine of most conchifera, and although the 

 nature of the food is not distinguishable in this soft digested 

 matter, small entomostracous Crustacea are often found within 

 the respiratory sac, and probably form a principal part of 

 their food. This dark matter filling the intestine renders 



