120 NEW YORK STATE MUSEUM 



4 Insecta : insects 



5 Arachnida: spiders, scorpions, mites etc. 



A part of the Crustacea are treated in the present catalogue. 



The name Crustacea imphed to early zoologists an animal with 

 a hard but flexible shell, in distinction from the Testacea in which 

 the shell was hard and brittle, as in the clam or oyster. The insects 

 and other groups have, however, a hard and flexible shell, so that 

 this will not serve as a distinction. 



On comparing the classes we find that a much greater portion 

 of the Crustacea than of the other groups is aquatic and the breath- 

 ing apparatus is of a different character. The insects, myriopods 

 and Peripatus breathe through a series of tubes or tracheae, which 

 carry the air to all parts of the body, while in the Crustacea there 

 are no tracheae and the blood becomes aerated either in certain 

 localized regions, the gills, or else over the general body surface. 



In completing the definition of the class it would be well for the 

 student to have before him some more or less familiar crustacean 

 and examine its structure. 



Take then the crayfish or lobster, which differ but little from each 

 other and are common though they can not be called typical 

 Crustacea in the sense that they exhibit crustacean characters in 

 their simplest form. 



First, we observe that the body is segmented. This is not so 

 obvious in the anterior part of the crayfish, for the head and thorax 

 are fused together into a cephalothorax, which is covered by a fold 

 of the integument, the carapace. The presence of appendages 

 shows, however, that this part of the body is composed of segments 

 fused together. The abdomen is distinctly segmented. 



The cephalic region or head in the crayfish, as in all Crustacea, is 

 composed of a prostomium and five segments, all indistinguishably 

 fused. It bears two pairs of feelers, the anterior of which are 

 known as the antennulae and the posterior as the antennae. Then 

 follow a pair of eyes, stalked, in the order to which the crayfish 

 belongs. On the underside of the head are the mouth parts which 

 consist of a pair of mandibles or jaws and two pairs of maxillae. 



