SECT, xii RELATION OF APUS TO CRUSTACEA 189 



of the Phyllopocla to the Crustacean leg, we have in 

 Limulus a small number of segments with a small 

 number of limbs, showing essentially the same 

 differentiation as we find in the legs of Apus, but more 

 specialised in adaptation to its manner of life. We 

 have anteriorly the more typical Crustacean limb, 

 posteriorly the more parapodial limb, the transition 

 between the two, however, being not gradual but 

 sudden. 



In trying to homologise the limbs of the two animals, 

 we shall have to utilise the conclusions arrived at in 

 Part I. 



The first pair of limbs of Limulus occupies a place 

 corresponding to that of the first pair in Apus, viz., 

 at the sides of or close to the labrum, and is homo- 

 logous with the first pair of antennae. In general 

 form these limbs do not differ much in the two animals, 

 as may be seen by comparing Fig. 43 and Frontis- 

 piece. The chief difference is that in Limulus the 

 bends have developed hinges, owing to the greater 

 development of the exoskeleton, and there are chelae 

 at the tips. We need hardly say that neither of 

 these points is of very great morphological worth. 

 For instance, within the Arachnoidea we find one 

 and the same limb, the pedipalp, in one group (the 

 Araneidae), as a simple feeler ; in another (the Scorpio- 

 nidae), it develops powerful chelae ; and within the 

 group of the Spiders themselves the tips of the pedi- 

 palps in many males undergo even more wonderful 

 modifications for the purposes of copulation. 



With regard to the second antenna, we were at first 



