24 THE APODID^ PART i 



of the limb should develop most strongly the 

 exoskeleton, and the convex side the hinges. 



We thus find in Apus the Annelidan cuticle 

 chaneine into the exoskeleton of the Crustacea ; the 



o o 



principles of the change being for the most part 

 easily deciphered. 



The importance of this gradual thickening of the 

 cuticle for the whole organisation can hardly be over- 

 estimated. As one of the special characteristics of the 

 Crustacea, useful at all stages, it is naturally very 

 early developed, the youngest larva having a cuticle 

 too thick to allow of gradual regular growth. This 

 leads to the habit of moulting, which was doubtless 

 very gradually acquired. The earliest thickenings 

 probably peeled off separately in flakes, as the areas 

 which they covered increased in size. Such half- 

 loosened flakes in all parts of the body would, however, 

 materially hinder the animal in the struggle for 

 existence, and natural selection would soon bring 

 about a shortening of the process, those animals being 

 most successful who were, during life, least encumbered 

 by loosening flakes, i.e. who threw them off altogether. 



But still more important consequences of the stiff 

 cuticle are to be traced in the inner organisation. 

 Some of the greatest differences between the anatomy 

 of Apus and that of an Annelid can be traced directly 

 to its development. 



The hairs with which the cuticle is covered will be 

 described in the section on the sensory organs. 



We thus find in their outer organisation that the 



