170 OEGANS OF DIGESTION". 



the development of the glandular organs connected with the 

 assimilating functions. In Holothuria we have seen salivary 

 vessels developed in the form of a series of blind processes 

 surrounding the mouth. In the mollusca these organs are 

 glandular, and extend through nearly half the body in Aplysia 

 (s, v) ; the liver in the mollusca is likewise glandular, whilst 

 in the articulated animals it is composed of a series of con- 

 voluted vessels. A rudimentary pancreas exists in some 

 mollusca, which, like the salivary vessels in Holothuria, 

 assumes the form of a long blind secreting sac. The intes- 

 tinal canal (s) in Pleuro-branchus and Aplysia presents 

 nothing very remarkable ; it makes several turns through 

 the structure of th liver, terminating in the rectum (), 

 which opens near the branchial, or respiratory aperture (cT) ; 

 the ovary (v), the oviduct (V) and its appendage (y) occupy 

 the posterior part of the body, surrounded by the testes (w) 

 and the epididymus (x) ; ascending from the latter is seen 

 the common generative canal (z, z) ; the heart, consisting of an 

 auricle (/3) and a ventricle (3), is placed near the branchiae 

 (B) ; the principal artery () runs forwards to supply the dif- 

 ferent organs situated at the anterior part of the body ; the 

 gastric artery (T) and the hepatic (T') artery are given oif from 

 the root of the principal trunk. 



In Bulla lignaria the plates lining the muscular sto- 

 mach, or gizzard, acquire the consistence of shell ; they 

 are moved by powerful muscles, and perform the part of 

 stomach jaws. Among the gasteropodous mollusca the liver 

 is a very voluminous organ, divided into many lobes, and 

 very distinct from the intestine ; thus, in the garden snail, 

 whelk, &c., it occupies the several turns of the shell, embracing 

 the convolutions of the intestine, and pouring its secretion, by 

 distinct ducts, into the cavity of the stomach. In the slug and 

 sea-hare it occupies a great portion of the muscular sac, 

 common to the general visceral cavity. The liver of the 

 Doris is remarkable, from the circumstance of possessing, 

 besides ducts for pouring the biliary secretion into the sto- 

 mach, a particular canal running in a direct course from the 

 liver to the anus, and conveying a portion of the bile out 

 of the system, without traversing the intestinal tube. This 

 anatomical fact clearly proves that a portion of the bile is 



