CONCHIFERA. 

 Fig. 345. 



(a, fig. 346) placed at Fig. 346. 



the anterior part of the 

 animal is deeply hid- 

 den between the foot 

 (b, Jig. 346), and the 

 anterior retractor mus- 

 cle (c) in the Dimyaria, 

 and under a kind of 

 cowl formed by the 

 mantle in the Mono- 

 myaria. The mouth is 

 in the form of a trans- 

 verse slit, comprised 

 between two lips, ge- 

 nerally thin and nar- 

 row, as in almost all 

 the Dimyaria, or lo- 

 bated and digitated, 

 as in some of the 

 Monomyaria, (a, Jig. 

 348). The lips ex- 

 tend on either side in the form of two flat- 

 tened smaller appendages, more or less elon- 

 gated, occasionally truncated, streaked or 

 laminated on their internal surface, and to 

 which the title of labial palps has by general 

 consent been given, (d,fj.g. 346, c,Jig. 348.) 



The mouth in the Conchifera never presents 

 any part that is hard. In the greater number 

 of these animals it terminates without any 

 intermediate passage in a stomach, the form 

 of which is subject to but little variety. 

 When there is an oesophagus (a, Jig. 347), it 

 is variable both in point of length and capacity ; 

 it has nothing constant, relatively to the other 

 distinctive characters of the groups established 

 among the conchifera generally : thus it either 

 occurs or is wanting indifferently among the 

 individual members of the dimyarian and mo- 

 nomyarian families. 



Fig. 347. 



The stomach (6, Jig. 347, d,fig. 348) is a 

 membranous pouch, commonly pear-shaped, 



