xn 



PHYLUM MOLLUSCA 



719 



is greatly reduced, thin and horn-like, and concealed within the 

 mantle, while in the 

 nudibranch members of 

 the same sub-order it 

 is entirely absent (Fig. 

 633). The shell is also 

 completely absent in 

 some of the pelagic 

 forms (Heteropoda and 

 Pteropoda) ; in others, 

 though present and ex- 

 ternal, it is too small to 

 enclose the animal (Fig. 634). In the slugs, among the Pulmonata, 

 the shell is vestigial and in most cases is concealed by the mantle 



(Fig. 635). 

 The foot varies in the extent of its 



development in the different families 



FIG. 632. Cypraea moneta (Cowrie). Showing the 

 mantle, provided with marginal tentacles, partly en- 

 veloping the shell. Br. siphon ; MM. mantle ; F. foot ; 

 T. tentacles at the edge of the mantle. (From Cooke, 

 after Quoy and Gaimard.) 



FIG. 633. Doris (Archidoris) 

 tuberculata. a. anus ; br. 

 branchiae ; m, penis ; rh, rh, 

 tentacles. (From the Cam- 

 bridge Natural History.) 



FIG. 634. Carinaria mediterranea. a. anus ; 

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

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

 Cambridge Natural History.) 



po 



of the class. It usually presents an elongated flat ventral surface 

 on which the animal creeps by wave-like contractions of the 

 muscular tissue. An exceptional case is that of Caecum, in which 



the creeping movement is 

 entirely due to the action of 

 cilia covering the ventral 

 surface. In the typical 

 Gastropods the foot is usually 

 distinguishable into three 

 parts, a middle part or meso- 

 podium, which is the most 

 important, with a smaller 

 anterior propodium and pos- 

 terior metapodium. In many 

 burrowing forms (Fig. 636) 



FIG. 635. A Slug (Umax). PO, pulmonary ,1 rnYvnorlinm is 



aperture (From the Cambridge Natural History.) ^ ne prOpO( im IS 



