154 THE APODID/E part i 



parapodium. That this is the true homology we 

 have httle doubt ; the facts that the second antenna 

 is a sensory Hmb, that its tip carries long sensory 

 hairs, that, as a long rowing foot, it requires to be 

 provided with a fine sense of touch, all tend to sup- 

 port it. It is difficult to say whether the thorn-like 

 process at the base of the limb represents the ventral 

 parapodium ; it is possible that, in order to facilitate 

 the motion of the rowing foot, the ventral branch has 

 disappeared, just as in the higher Crustacea, when the 

 legs become more specialised as such. 



Figs. 35 and 36 are two views of the Nauplius of L 

 productus. In these the structure of the whole limb 

 in the manner we have described is particularly clear, 

 the sensory nature of the larger branch of the second 

 antenna being marked by the length of its filaments. 



The further development of the limb is interesting. 

 As it ceases to be a rowing limb and to be specialised 

 as a sensory organ, one of its branches, that represent- 

 ing the tip of the original dorsal parapodium, dege- 

 nerates, leaving the other, the sensory cirrus, i.e. the 

 exopodite, to form the distal portion of the limb. In 

 Apus a small rudiment of the endopodite remains 

 (see Fig. 7 B, p. 34, where the lettering explains the 

 homologies). 



If the thorn-like process at the base of the 2nd 

 antenna is really the homologue of the ventral para- 

 podium, we may perhaps see in it an attempt on the 

 part of the very first Crustacean to use the ventral 

 parapodia of all the segments round the mouth for 

 mastication, an effort which succeeded well in Limulus. 



