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. [Illustration: FIG. 100.--_Yoldia limatula_ Say, Greenland, showing the short plumed branchiae (_br_, _br_), the gasteropodous foot (_f_), and the large labial palps (_l.p_, _l.p_): =A=, as seen from the ventral margin; =B=, from the left side, with the mantle turned back; _a.m_, position of anterior adductor muscle; _i_, intestine; _l_, liver; _m_, _m_, mantle.] The _byssus gland_, secreting a byssus of horny threads, is characteristic of many Pelecypoda, and may be observed by any one in the common mussel. It occurs in the larvae of many species which do not possess a byssus in the adult stage. The pedal gland of many Prosobranchiates, which secretes a tough and almost thread-like slime, is possibly homologous with the byssus gland of bivalves. =Nervous System=