A CRAYFISH OR A LOBSTER 21 



Class : Crustacea. Division : Malacostraca. Order : Decapoda 

 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 anatomy 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 animal. 

 It is bilaterally symmetrical ; the body is composed of a number 

 of serially arranged segments, which are called somites, or meta- 

 meres ; 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, that is, 

 each somite, or metamere, bears a pair of appendages ; the ante- 

 rior 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 these features are equally 

 characteristic of insects and myriapods. 



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

 falls into three distinct divisions : the head, thorax, and abdo- 

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

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

 mon 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 segmenta- 

 tion, because of the fusion of the somites and the presence of a 

 hard, shieldlike structure covering it, which is called the cara- 

 pace, but on the ventral side the segmentation is distinctly seen. 

 Extending along the entire ventral surface of the animal are the 

 paired appendages. Their metameric significance may not be 

 seen in the cephalothorax, but it will be distinctly seen in the 



