CONCHIFERA. 



697 



sometimes globular, rarely elongated and 

 narrow. When the esophagus exists, it opens 

 into the upper part of the stomach ; but 

 when that canal is absent, the mouth termi- 

 nates directly in the stomach. Examined 

 internally, the stomach presents several de- 

 pressions irregularly dispersed over its surface, 

 by means of which the bile is brought into 

 its cavity ; it is on this account that these 

 minute depressions have received the name 

 of the biliary crypts. The intestine (r, jig. 

 347, e, Jig. 348) arises from the posterior 

 wall of the stomach, and a very singular ap- 

 paratus is occasionally found in its vicinity 

 (d, fig. 347), the use of which is not yet de- 

 termined. It consists of a small appendage 

 which may be compared to the vermiform 

 process of the ccecum in the higher animals ; 

 it communicates with the stomach, and is filled 

 by a horny process or stylet of different lengths 

 and thickness, according to the genera and 

 species examined. The anterior extremity of 

 this body is attached to the parietes of the 

 stomach by means of small extremely thin and 

 irregular auricular processes (oreillettes). It is 

 to be presumed that quantities of the food may 

 fall during the act of digestion between the 

 parietes of the stomach and the horny body, by 

 it to be pressed or bruised in some particular 

 manner. Yet when those conchiferous ani- 

 mals which are furnished with the apparatus 

 just mentioned, are examined by dissection, 

 no particle of food is found in such a position. 

 We may therefore be allowed to conjecture 

 that this part accomplishes some other purpose 

 in the economy of the conchifera. Whatever 

 this may be, it must, we should imagine, be 

 connected with the function of digestion. 



The intestinal canal in the conchiferous 

 Mollusca is generally slender, cylindrical, and 

 from one extremity to the other almost always 

 of the same diameter. After having made a 

 variable number of convolutions within the 

 substance of the liver and the ovary, the in- 

 testine comes into relation with the dorsal and 

 median line of the animal's body. It con- 

 tinues in this direction to the posterior extre- 

 mity, there to terminate in the anus (e,jig. 347, 

 ,/i Jig- 348) ; the whole of this dorsal part of 

 the intestine is named rectum. The rectum is 

 generally longer in the Dimyaria than in the 

 Monomyaria, because the anus is found above 

 the superior adductor muscle in the former, 

 whilst in the Monomyaria the rectum twists 

 round behind the central muscle to terminate 

 in an anus which floats between the edges of 

 the mantle. 



The liver (j\ jig. 347, g, fig. 348) is a 

 bulky organ enveloping the stomach and part 

 of the intestine. It pours the product of its 

 secretion directly into the stomach by means 

 of the biliary crypts. The liver alone con- 

 stitutes a very large portion of the visceral 

 mass, and consequently of the body of the 

 animal; it consists of a great number of fol- 

 licles connected together by means of lax and 

 extremely delicate cellular membrane; this 

 structure renders the organ very easily torn. 

 We shall see by-and-tye that it is traversed in 



VOL. i. 



Fig. 348. 



8 



the greater number of mollusks by several 

 muscles belonging to other parts, an arrange- 

 ment which contributes to support and give it 

 greater strength. 



The exposition which has now been given 

 of the structure of the organs of digestion, 

 affords a ready explanation of all that bears 

 upon this function in the conchiferous mol- 

 lusca. These animals not having the mouth 

 armed with any hard part are unable to seize 

 and swallow any kind of solid food, so that in 

 general nothing more is found in their sto- 

 machs than segregated particles, proceeding 

 without doubt from the decomposition of 

 aquatic animals and plants. The lips, and 

 unquestionably the labial palps also, are de- 

 stined to give the animal perception of the 

 aliment it takes. Once in the stomach, this 

 aliment, impregnated with bile and probably 

 also with a gastric juice secreted by the lining 

 membrane of this pouch, is subjected to a 

 first digestive elaboration ; it next passes the 

 pylorus when it exists, and then traverses the 

 intestinal canal and supplies to the absorbent 

 system the elements necessary to the nutrition 

 of the animal. 



It does not appear that there is any par- 

 ticular system of absorbent vessels in the con- 

 chiferous Mollusca; the veins perform the 

 office of absorbents, and they transmit with- 

 out any intermedium, and without their under- 



2 z 



