230 



THE APODID^ 



"PART 11 



those of either Apus or Limiilus, but by closer study, 

 and by referring them back to the original Annelidan 

 parapodia, their common origin becomes evident. 

 We find the limbs much specialised, the habits of 

 life of the animal leading to certain modifications. 

 In the first place, the creeping motion along the 

 ground required the development of legs. In the 

 second place, the habit of rolling up requires that 

 the limbs should take up as little room as possible, 



Fig. 54. — Restored transversed section through Calymene Senaria (after Walcott), 

 showing the spiral gills, the exopodite (= the sensory cirrus of the dorsal para- 

 podium), the endopodite, or ambulatory foot (= the dorsal parapodium), and the 

 thit^h piece or coxal joint, the ventral projection of which corresponds with the 

 ventral parapodium, cf. Fig. 49. 



first, to render the rolled-up attitude mechanically 

 possible, and second, in order that as much of the 

 respiratory medium as possible may be enclosed. 

 The special form of the limbs can thus be under- 

 stood. We find (Fig. 54) a large basal joint, the 

 inner ventral part of which is almost certainly to be 

 homologiscd with the ventral parapodium of the 

 Annelid. This again originally functioned as a 

 gnathobase or accessory jaw for the holding and 

 forwardinsf of food to the mouth, i.e. on a certain 



