30 THE APODID/E PARTI 



of their absence in Apus, where we find many 

 thickenings of the cuticle which we regard as an 

 cxoskeleton not in the act of disappearing but in 

 that of appearing for the first time. 



The fact that the limbs are little more than folds 

 of the integument, like Annelidan parapodia, is fully 

 borne out by the examination of their musculature, 

 which will be described in detail later. (Sect. IV.) 



(2) The course of a line traced through the bases 

 of the limbs of Apus agrees well with that of a 

 similar line drawn through the parapodia of our 

 imaginary bent Crustacean-Annelid. Commencing 

 at the anterior antennae (see Figs. I and 2) at the 

 side of the prostomium or upper lip, this line passes 

 in both cases vertically upwards and (for reasons 

 to be given later, see p. 212) slightly outwards ; 

 passing through the 2nd antennae, it bends round 

 to run backwards, trending, however, gradually 

 towards the ventral surface. The close agreement 

 between the courses of these two lines is, morpho- 

 logically, a fact of great significance. The more 

 ventral trend of the line in the posterior end of 

 Apus was to be expected as a necessary adaptation 

 to the Crustacean manner of life, i.e., to the use 

 of the appendages as limbs whose functions were 

 primitively all directed towards the middle line. 

 The position of the antennae is especially interest- 

 ing. In the Annelid, these antennae were originally 

 metastomial, but have become prostomial by the 

 bending of the segments on one another. The 

 assumption of the rise of the Crustacean head from 



