520 PHILOSOPHY OF ZOOLOGY. 



of bone, or appears as a calcareous crust in the crabs. In 

 some species it is elastic, in others brittle. 



The appendices of the skin consist of spines, hairs, and 

 scales. The spines ai'e meiely projecting portions of its 

 substance, and are usually distributed over certain parts of 

 the feet, to aid the locomotive powers. Hairs are often dis- 

 tributed over the whole body ; and, while they pass into 

 spines on the one hand, they become, on the other, so ex- 

 ceedingly fine as to require the aid of a powerful magnifier 

 to trace their character. These spines and hairs, being 

 merely elongations of the skin, are not easily rubbed off. 

 It is otherwise with scales. Some of these are inserted into 

 their skin at their proximal, and are free at their distal ex- 

 tremity, and in some insects are so feebly connected, as to 

 fall off, in many species, by touching them with the finger. 

 These scales, in the butterfly, bear a remote resemblance 

 to feathers in their form, and are very extensively used as 

 pleasing objects for the microscope. 



In those insects, which undergo several changes of form 

 during life, the cutaneous system, in the first periods, pos- 

 sesses considerable powers of production. In the cater- 

 pillar state, the skin is cast or changed, several times, and 

 along with it, the spines and hairs by which it is covered. 

 That which is cast off bears a resemblance to the cuticle in 

 the perfect animals ; but the skin which remains to supply 

 its place, is similar in its structure. When the insects ar- 

 rive at their last stage, or that of maturity, the reproduc- 

 tive power of the skin does not seem to be exerted. Nei- 

 ther holes in the wings, nor fractures in the joints, appear 

 to be repaired. Among the crabs and spiders, the casting 

 of the skin takes place periodically. 



Comparative anatomy has hitherto failed in detecting any 

 glands subservient to the functions of tlie skin. As the 

 aquatic insects, however, are never wet with water in which 



