SECT, xi DEVELOPMENT 155 



as we shall see in Part II. In the other Crustacea, 

 however, the greater efficiency of the ventral para- 

 podia of the 3rd, 4th, and 5th segments, owing to 

 their easier concentration round the mouth, led to 

 their specialisation as mandibles and 1st and 2nd 

 maxillae, so that the masticatory process of the second 

 antenna was rendered useless and disappeared (see 

 table p. 250). 



The third limb has again essentially the same shape 

 as the second. We have the dorsal (and ventral ?) 

 parapodia, with an appendage on the former homo- 

 logous with the sensory cirrus or the antennal branch 

 of the second limb. The dorsal parapodium gradually 

 disappears in Apus, leaving only the ventral as masti- 

 catory ridge or mandibles. It is however retained 

 as palp in the higher Crustacea. 



We repeat then here what we have learnt from our 

 study of the limbs of the adult Apus and of those of 

 the Nauplius larva. The tip of the dorsal Annelidan 

 parapodium forms the endopodite of the Crustacean 

 limb, the sensory cirrus the exopodite, and the ventral 

 parapodium the masticatory process. Applying this 

 once more to the trunk legs of Apus, we conclude that 

 the flabellum becomes the exopodite, and the limb 

 proper (i.e. the dorsal parapodium) is the endopodite ; 

 the gnathobase or first endite is the ventral para- 

 podium, which in the typical trunk limb of the 

 Crustacea disappears, but may be retained as a 

 primitive feature, as in Apus, Limulus, and the 

 Trilobites, and, as on the maxillipedes of the higher 

 Crustacea, as a masticatory process. 



