68 



THE GASTROPODA 



tion of the summit of the spire, in Stylifer and allied parasitic 

 forms (Fig. 20). 



1. The Foot. This is primitively and normally formed by a 

 powerful mass of ventral muscles with a more or less elongated 



ventral creeping surface (Fig. 44, B). But this primitive condition 

 may be modified in relation to different conditions of existence. 

 Thus, among sedentary Gastropods the foot is reduced to a simple 

 discoidal prominence in such fixed forms as Vermetus (Fig. 45) and 



Fio. 45. 



Vermetus triqueter, with broken shell ; left-side view, co, columellar muscle ; /, foot ; ?, 

 niouth; o, aperture of the shell ; op, operculum ; ov, eggs ; pa. s, mantle slit ; p.t, pedal tentacles; 

 sh, shell. (After Lacaze-Duthiers.) 



Magilus. In Bathysciadium (Fig. 126) the ventral surface of the 

 foot assumes the form of a sucker, the central portion of which is 

 covered by a thick cuticle, and the circumference is ciliated. In 

 the parasitic forms Stylifer and Thyca the foot is atrophied and is 



