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 transverse, 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 _Strombus_ (Fig. 99) it becomes erected into a sort of hump or column, on the top of which the operculum is situated. [Illustration: FIG. 98.--_Oliva textilina_ Lam., showing how the front part of the foot (_f_) is developed into a sort of fender, the propodium (_pr_); _e_, _e_, eyes; _m.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 _Haliotis_ 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 tentaculae of _Monodonta_ (Fig. 82, p. 178), and of other sub-genera of the Trochidae.[311] 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 enabled to progress through the water. The paired natatory lobes of the Pteropoda are simply the parapodia of the Tectibranchs modified for swimming purposes. [Illustration: FIG. 99.--_Strombus lentiginosus_ Lam., showing the modified form of the foot (_f_): _e_, _e_, eyes on their pedicels; _mp_, metapodium; _op_, operculum; _p_, penis; _pr_, proboscis; _t_, _t_, tentacles. (After Quoy and Gaimard.)]