XII 



PHYLUM MOLLUSCA 



687 



comes to be enclosed in a complete sac of the mantle ; such 

 enclosed shells are 

 always imperfectly 

 developed and incap- 

 able of covering the 

 body. Thus in Aply- 

 sia and some other 

 Opisthobranchs the 

 shell is greatly re- 

 ducJtffc thin and 

 hor^Bind concealed 

 witlJJl the mantle, 

 while in the nudibranch (Fig. 585) members of the same sub- 

 order it is entirely absent. The shell is also completely ab- 

 sent in some of the pelagic forms 

 (Het&ro'poda, and Pteropoda); in others, 

 though present and external, it is too 



-M 



FIG. 584. Cyprsea moneta (Cowrie). Showing the mantle, 

 provided with marginal tentacles, partly enveloping the 

 shell. SI. siphon ; M.M. mantle ; F. foot ; T', tentacles at the 

 edge of the mantle. (From Cooke, after Quoy and Gaimard.) 



FIG. 585. Doris (Archidoris) 

 tuberculata. a. anus ; l;\ 



branchiae; m, penis; //<. //<, 

 tentacles. (From the Ca,nh,-''<i>.i< 

 S< it i' i ~((l History. ) 



FIG. 586. Car in aria mediterranea. a. anus; 



br. branchia ; /. foot; i. intestine; m. mouth; p. 

 penis ; s. sucker ; sh. shell ; t. tentacles. (From the 

 Cambridge Natural History.) 



PO 



.small to enclose the animal (Fig. 586). In the slugs, among the 

 Pulmonata, the shell is vestigial and concealed by the mantle 



(Fig. 587). 



The foot varies in the 

 extent of its development 

 in the different families of 

 the class. It usually pre- 

 sents an elongated flat 

 ventral surface on which 

 the animal creeps by wave- 

 like contractions of the 

 muscular tissue. In the 

 typical Gastropods the foot 

 is usually distinguishable 



middle 



