PROPODIUM AND MKTAl'ODIUxM 



199 



the little lap])ets at the end of the 'tail' ])rol)a1)ly serve as 

 rudders. 



In JiL'hiiiqiiix and redlj^)cs the propodiuin is marked oft" hy 

 a groove across the ventral surface. When the animal is in 

 motion it first advances the propodiuni and then pulls the rest 

 of the foot after it witli the looping gait of certain caterpillars. 

 \n many C'yelostomatidae this groove, instead of heing transverse, 

 is longitudinal, and the animal advances first the right and then 

 the left segment o\' the foot, which gives it a swaying motion 

 from side to side. 



Upon the metapodium lies the operculum, when it occurs. 

 As a. rule the metapodium is not sharply marked off from the 

 rest of the foot. In l^irifinhuH (Fig. 99) it Ijecomes erected 



/' 



Fig. 9S. — Ollca fcxtilijuc Lam., showing how the front part of tlie foot (/) is developed 

 into a sort of fender, the pro]iodiuni (pr) ; e, e, eyes ; m.cqt, front api^endage of 

 mantle ; m.ap', hinder api)en(Lage of mantle, folded into the snture wlien thu 

 animal is at rest : s/, siphon ; t, t. tentacles. (After Qnoy and Gainiard.) 



into a sort of hump or column, on the top of which the opercu- 

 lum is situated. 



The epipodiiiiii is a ]irominent fold or horder, which occurs 

 upon the u])per edge of the foot in most Diotocardia. In 

 Halioti>i it is of eonsideral)le hreadtli, and is covered l)y a. 

 number of lobes which spring i'rom a moss-like prolongation of 

 tlie skin. From the epipodium are developed the lateral tenta- 

 culae oi Monoclonta (Fig. 'S'l, p. 178), and of other subgenera of 

 the Trochidae.^ 



In the Opistho])ranchiata the lateral edges of the foot (the 

 'iJarcqwdia) are frequently produced into liroad folds or wing- 

 like extensions, whicli in many cases tend to fold over the shell, 

 and, in conjunction witli the mantle, eventually imbed it 

 altogether. By the wav}- motion of the parapodia the animal is 



^ Compare Pelseneur, BuJl. Sri. Ft: BrJg. (3) xi.x. pp. 107, \9'i. 



