200 



MODIFICATIONS OF THE FOOT 



enabled to progress through the water. The paired natatory 

 lobes of the Pteropoda are simply the parapodia of the Tecti- 

 branchs modified for swimming purposes. 



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 Oxy gyrus and Atlanta^ for 

 instance, the propodium and metapodium are sharply distin- 

 guished 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. 



—p 



Fig. 99. — Strombus lentigi- 

 nosus Lam., showing the 

 modified form of the foot 

 (/) : e, e, eyes on their 

 pedicels ; mp, metapodium ; 

 op, operculum; p, penis; 

 pr, proboscis ; t, t, tentacles. 

 (After Quoy and Gaimard.) 



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 pro- 

 podium. In the Cephalopoda the arms and funnel represent 

 the modified foot, the sides of which are prolonged into a num- 

 ber 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 gradu- 

 ally 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. 



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