CUTICULAR TISSUE. 197 
morphosis and consequent destruction of the 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 blood; while 
the distinctive structural characters of the epiostracum, 
the ectostracum, and the endostracum, are the results of 
a process of metamorphosis which goes on pari 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 physical 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 processes which exhibit every gradation from a fine 
microscopic down to stout spines. As these processes, 
though so similar to hairs in general appearance, are 
essentially different from the structures known as hairs 
in the higher animals, it is better to speak of them as 
sela@. 
These sete (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 sete, the axis gives off slender lateral 
