XII 



PHYLUM MOLLUSCA 



739 



or mesopodium, which is the most important, with a smaller 

 anterior propodium 

 and posterior meta- 

 podium. In many bur- 

 rowing forms (Fig. 

 626) the propodium 

 is well developed and 

 sharply marked off to 

 act as a burrowing 



organ. In a few cases 



- /' . . -I i _ m , 



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



FIG. 622. Cypraea moneta (Cowrie). Showing the mantle, 

 . provided with marginal tentacles, partly enveloping the 

 shell. Br. siphon ; M. M. mantle ; F. foot ; T. tentacles at the 



a pair of tentacles 



the pedal tentacles 



are situated at the anterior end of the foot ; still rarer is a pair 



of similar appendages at the posterior end. The whole foot 



becomes reduced in the few Gastropods 

 that remain fixed. The metapodium 

 very usually in the Streptoneura bears 



FIG. 623. Doris (ArcMdoris) 

 tuberculata. a. anus ; lr. 

 branchiae ; ;/;, penis ; rh, rh, 

 tentacles. (From the CambrUgt 

 Natural History.) 



Fir.. 624. Carinaria mediterranea. a. anus; 

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

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

 Cambridge Natural History.) 



Pfl 



a disc or stopper the opcrculum already referred to usually 

 horn-like, rarely completely calcified, more commonly horn-like with 



a thin calcareous invest- 

 ment by means of which 

 the aperture of the shell is 

 closed when the animal is 

 retracted. 



In some forms, such as 

 the Sea-hares (Aplysia, Fig. 

 627), the foot develops a 

 pair of lateral lobes the 

 parapodia which act as 

 fins ; and in the Ptero- 

 pods (Fig. 628) which are 



FIG. c.2o. A Slug (Limax). PO, pulmonary 'II J' J ? 



aperture. (From the Cambridge Natural Hiitonf.) Specially modified IOr a 



