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.
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.
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.)
It is in the Heteropoda, Pteropoda, and most of all, the Cephalopoda, groups which have, for the most part, exchanged a crawling for a swimming life, that the modifications of the foot are most considerable. In Oxygyrus and Atlanta, for instance, the propodium and metapodium are sharply distinguished from the mesopodium, and no doubt have acquired, as a means of propulsion, the power of separate movement, the animal swimming with these portions of the foot uppermost. In Carinaria and Pterotrachea the metapodium has probably become continuous with the long axis of the body, while the so-called ‘foot’ with its sucker represents only the original propodium. In the Cephalopoda the arms and funnel represent the modified foot, the sides of which are prolonged into a number of very long specialised tentaculae. In the adult Cephalopod some of the arms have assumed a position in advance of the mouth, the latter being in fact surrounded by a circle of arms. But in the Cephalopod embryo the mouth opens as in the Gasteropoda, i.e. in advance of the arms, and it is only gradually that it becomes encircled by them. Arms and funnel alike are found to be innerved from the pedal ganglion.[312]
The pointed axe-shaped foot, which is characteristic of the majority of Pelecypoda, is doubtless derived from a form more akin to the flattened ‘sole’ of the Gasteropoda. A foot with something of this disc-shaped base actually occurs in some of the Nuculidae, the parapodia being furnished with pleats which recall similar formations in other Orders (Fig. [100]). The principal modifications of the foot are due to its employment as a burrowing organ. In genera which burrow but slightly it is small and feebly developed, while in genera which habitually excavate, it becomes the largest and strongest organ of the body. At the same time it has a tendency to shift its position from the ventral to the anterior margin, accompanied by a corresponding narrowing of the shell, until it arrives at the position seen in Mollusca of the shape of Mya, Pholas, and Solen. In sedentary or attached genera, e.g. Pecten, Chama, Ostrea, the foot tends to become aborted.