368 mVERTEBBATE MOBPHOLOGT. 



CHAPTEE XIII. 



TYPE CRUSTACEA. 



The group Crustacea includes a very large number of 

 forms, most of which are marine, though many are found in 

 fresh water and a few are even terrestrial. A great diversity 

 of form is found in the various members of the group, but at 

 the same time the general structure, except in. forms degen- 

 erated by parasitism, shows comparatively close similarity 

 throughout. 



The body is enclosed in a thick chitinous cuticle which 

 not infrequently becomes hardened by the deposition of 

 calcareous matter in it, producing what may almost be con- 

 sidered a shell and giving origin to the name applied to the 

 group. This covering serves not only for protection, but also 

 as a point d'appui for the insertion and origin of muscles. 

 Where it reaches a considerable thickness it becomes more or 

 less regularly divided into segments, separated by intervals in 

 which the cuticle remains thin, so that movement of the 

 various segments upon each other are possible. 



As a rule there is attached to the sides of each of these 

 segments an appendage, also inclosed in a more or less thick 

 cuticle and jointed, this jointed character having suggested 

 the reference of the Crustacea together with the Arachnida 

 and Tracheata to a single group termed the Arthropoda. An 

 examination of the internal parts, especially of the nervous 

 system, shows that these various body-segments are in reality 

 metameres, and that the Crustacean is, like the Annelid, a 

 metameric organism. A characteristic of the Crustacea, how- 

 ever, is a tendency towards a greater differentation and con- 

 solidation of the metameres than is found in the Annelida, a 

 tendency especially well marked in the anterior region of the 

 body, where a varying number of the metameres fuse more or 

 less perfectly together to form a distinct head, bearing the 



