156 THE APODID^ . PARTI 



We repeat further what was stated on p. 50, that 

 theoretical considerations would also lead one to 

 expect a retention of the parts mentioned to form the 

 Crustacean limb, the capacity of sensation being 

 necessary to all co-ordinated movement. Hence, as 

 the dorsal parapodium lengthened into a seizing foot 

 or locomotory organ, it had everything to gain by 

 retaining its sensory appendage. It is interesting to 

 note that when the exoskeleton is so developed that 

 the limbs are protected by an almost stony covering, 

 and the limb used simply for walking, the exopodite 

 disappears, while on the other hand it is nearly always 

 present in soft-skinned limbs, and generally seems to 

 have retained its sensory functions. As an instance 

 of this we can compare the thoracic with the abdominal 

 limbs of the macrurous Decapoda. 



When we come, in Part II., to consider the relation 

 of Apus to Limulus and the Trilobites, we shall find 

 considerable confirmation of the homologies here put 

 forward, the homologising of the limbs of these animals 

 with those of Apus being by no means the impossible 

 task it is too often assumed to be. 



2. The Unpaired " Eye!' — The presence of powerful 

 rowing limbs in the larva necessitated some more per- 

 fect sensory organ than any possessed by the Anne- 

 lidan larva ; hence the early development of the un- 

 paired " eye " which in the Nauplius probably still 

 retains its visual functions, although these have appa- 

 rently been lost in the median "eye" of Apus. The 

 structure of this organ in Apus has been described, and 

 its probable origin out of the two anterior eye-spots on 



