VII PROPODIUM AND METAPODIUM 1 99 



through it ; the little lappets at the end of the ' tail ' probably 

 serve as rudders. 



In Melampus and Pedipes the propodium is marked off by 

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

 motion it first advances the propodium and then pulls the rest 

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

 In many Cyclostomatidae this groove, instead of being trans- 

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

 and then the left segment of 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 Stromhus (Fig. 99) it becomes erected into 

 a sort of hump or column, on the top of which the operculum is 

 situated. 



Fig. 98. — Oliva textilina Lam., showing how the front part of the foot (/) is developed 

 into a sort of fender, the propodimii (pr) ; e, e, eyes; ni.ap, front appendage of 

 mantle; m.ap', hinder appendage of mantle, folded into the suture when the 

 animal is at rest ; si, siphon ; t, t, tentacles. (After Quoy and Gaimard.) 



The epipodium is a prominent fold or border, which occurs 

 upon the upper edge of the foot in most Diotocardia. In 

 Saliotis it is of considerable breadth, and is covered by a 

 number of lobes which spring from a moss-like prolongation of 

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

 culae of Monodonta (Fig. 82, p. 178), and of other subgenera of 

 the Trochidae.^ 



In the Opisthobranchiata the lateral edges of the foot (the 

 parapodia) are frequently produced into broad folds or wing- 

 like extensions, which in many cases tend to fold over the 

 shell, and, in conjunction with the mantle, eventually imbed it 

 altogether. By the wavy motion of the parapodia the animal is 

 ^ Compare Pelseneer, Bull. Sci. Fr. Belg. (3) xix. pp. 107, 182. 



