XII 



PHYLUM MOLLUSCA 



. 



T 



^ * 



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 1 



o 



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 

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



organ. In a few cases 



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 



T>r 



lr 



FIG. 623. Doris (Archidoris) 

 tuberculata. a. anus; lr. 

 branchiae ; in, penis ; rh, rk, 

 tentacles. (From the Ca,nbri<l<i> 

 Natural History.) 



Tit 



FIG. 624.^Carinaria mediterranea. a. anus; 

 l>r. branchia ; /. foot; /. intestine; m. mouth; />. 

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

 Camlr'nhje Natural History.) 



PO 



a disc or stopper the opcrwdum 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. 625. A Slug iLimax). PO, pulmonary 11 -,-n i f 



aperture. (From tlae Cambridge Natural History.) Specially modified IOr a 



