CUTIOULAR TISSUE. 197 



moi-phosis and consequent destruction of tlie cells to 

 which it owes its origin. 



The calcareous salts by which the calcified exoskeleton 

 is hardened can only be supplied by the infiltration of a 

 fluid in which they are dissolved from the Idood; while 

 the distinctive structural characters of the epiostracum, 

 the ectostracum, and the endostracum, are the results of 

 a process of metamorphosis which goes on i^ari passu with 

 this infiltration. To what extent this metamorphosis is 

 a properly vital process ; and to what extent it is explic- 

 able by the ordinary physk-al and chemical properties of 

 the animal membrane on the one hand, and the mineral 

 salts on the other, is a curious, and at present, un- 

 solved problem. 



The outer surface of the cuticle is rarely smooth. 

 Generally it is more or less obviously ridged or tubercu- 

 lated ; and, in addition, presents coarser or finer hair- 

 like j)rocesses which exhibit every gradation from a fine 

 microscopic down to stout spines. As these x^rocesses, 

 though so similar to haii's in general appearance, are 

 essentially different from the structures known as hairs 

 in the higher animals, it is better to speak of them as 

 setce. 



These sette (fig. 56, F) are sometimes short, slender, 

 conical filaments, the surface of which is quite smooth ; 

 sometimes the surface is produced into minute serra- 

 tions, or scale-like prominences, disposed in two or more 

 series ; in other setse, the axis gives off slender lateral 



