XII 



PHYLUM MOLLUSCA 



739 



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

 anterior propodium 

 and posterior mda- 

 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 

 i7i_ ^:^^^t7^^ ,yj^ that remain fixed. The metapodium 



very usually in the Streptoneura bears 



Fig. 1122.— Cypraea moneta (Cowrie). Showing the mantle, 



yi provided with marginal tentaclea, partly enveloping the 



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



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



Fio. 623.— Doris (Archidoris) 

 tuberculata. a. anus ; lir. 

 branchiae ; m, penis ; rh, rh, 

 tentacles. (Prom the Camlirnhje 

 Natural Hutorii.) 



Fio. (i24.— Carinaria mediterranea. a. anus; 

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

 penis : s. sucker ; sh. shell ; i. tentacles. (From the 

 C\(iiihrid(je Natural Histoi-T/.) 



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 

 farapodia — which act as 

 fins ; and in the Ptero- 

 pods (Fig. 628) which are 



Pio. 02.J.— A Slug (Umax). PO, pulmonary • ii T£ j e 



aperture. (From the Camhridge Natural Hisloi-y.) Specially moainecl lOr a 



