200 



MODIFICATIONS OF THE FOOT 



enabled to progress through the water. The paired natatory 

 loLes of tlie Pteropoda are simply the parapodia of the Tecti- 

 branchs modified for swimming purposes. 



It is in the Hetero]3oda, Pteropoda, and most of all, the Cephalo- 

 poda, groups which have, for the most part, exchanged a crawling 

 for a swimming life, that the modifications of the foot are most 

 considerable. In Oxygijrvs and Atlanta, for instance, the 

 propodium and metapodium are sharply distinguished from the 

 mesopodium, and no doubt have acquired, as a means of pro- 

 pulsion, the power of separate movement, the animal swimming 

 with these portions of the foot uppermost. In Cariiiaria and 



Fig. 99, — Stromhus lentigi- 

 nosus Lam., showing the 

 modified form of the foot 

 (/) : (', e, eyes nn their 

 pedicels ; mp, metapodium ; 

 op, operculum ; p, penis ; 

 jrr, proboscis ; t, t, tentacles. 

 (After Quoy and Gaimard. ) 



rtcrotrachca the metapodium has probal^ly become continuous 

 with the long axis of the l)ody, wlule the so-called ' foot ' with 

 its sucker represents only the original propodium. In the 

 Cephalopoda tlie arms and funnel represent the modified foot, 

 the sides of wliich are prolonged into a numljer of very long 

 specialised tentaculae. In the adidt 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 Casteropoda, 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.^ 



^ Pelseneer, Arch. Biol. viii. p. 723. 



