22 INVERTEBRATE ZOOLOGY 



A MACRURAN DECAPOD. A CRAYFISH OR A LOBSTER 



These two animals are very common, the one in fresh and 

 the other in salt water. In external form and internal anat- 

 omy they are exceedingly similar to each other, and the same 

 directions for dissection may be made to apply to either. In 

 habits and general method of life the animals also resemble 

 each other ; they move about at or near the bottom of the 

 water, preferring regions which are rocky or stony, and feed 

 upon small animals of all kinds and upon carrion. 



Observe the shape, color, and external anatomy of the ani- 

 mal. It is bilaterally symmetrical ; the body is composed of a 

 number of serially arranged segments, which are called somites 

 or metameres ; the dorsal and the ventral sides of the body are 

 unlike, the latter being characterized by the possession of a 

 series of paired and jointed appendages metamerically arranged; 

 i.e., each somite or metamere bears a pair of appendages ; the 

 anterior and the posterior ends are also unlike, the former being 

 characterized by the possession of organs of special sense and 

 the mouth. The external covering of the body is a chitinous 

 cuticula which constitutes an exoskeleton. All of these fea- 

 tures are equally characteristic of insects and myriopods. 



As in all crustaceans, and also in insects, the body of the 

 animal falls into three distinct divisions, the head, thorax, and 

 abdomen. The first two of these body-divisions do not, however, 

 articulate freely with each other as they do in insects, but, in 

 common with all the higher crustaceans, they are fused together 

 and form a single structure, which is called the cephalothorax. 

 The dorsal and the lateral surfaces of this division show no 



