MI 



PHYLUM MOLLUSCA 



F39 



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

 anterior propodium 

 and posterior mcta- 

 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 

 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 

 i*. ^a^tete^ ,-fL that remain fixed. The metapodium 



very usually in the Streptoneura bears 



Fio. 022.— 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. (Prom Cooke, after Quoy and Gaimard.) 



Pi ... B8.— P orta (Archidoris 

 tuberculata. a. anus ; br. 

 braachte; <«, jxjnis ; rh, rh, 

 tentacles. (From the t'ainbriilge 

 Sut, nil History.) 



Fin. Carinaria mediterranea. a. anus; 



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

 penis ; ». sucker ; ah. shell ; t. tentacles. (From the 

 ( i inil n- idy t Natural History.) 



a disc or stopper — the operculum 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 



P». 825.— A Slug (Umax). PO, pulmonary QT , AP ir,llv moHifipd for a 

 aperture. (From the Cambridge Natural Hittory.) Specially mOUineU IOl d 



