286 



ZOOLOGT. 



Articulata of Cuvier as a heterogeneous assemblage of 

 forms embracing at least three branches of the animal king- 

 dom, namely, the Vermes, 

 Tunicata, and Arthropo- 

 da. 



The Arthropoda are di- 

 vided into two well-de- 

 fined classes, i. e., the 

 Crustacea with two body- 

 regions, the head-thorax 

 a, and abdomen (Fig. 325), 

 and breathing through 

 the body- walls or by external gills ; and the Inseda ( Ti'a- 

 cheata), in which the arthromeres are grouped into three 

 body-regions, i. e. , a head, thorax, and abdomen ; and which 

 breathe by internal air-tubes (tracheas). 



Fig. 225.— Common Shrimp, Crangon. 

 cephalo-thorax ; b, abdomen. 



Class I. — Crustacea {Water-fleas, Shrimps, Lohsters, 

 and Crabs). 



General Characters of Crustaceans. — The typical forms 

 of this class are the craw-fish, lobster, and crab, which the 

 student should carefully examine as standards of comparison, 

 from which a general knowledge of the class, which varies 

 greatly in form in the different orders, may be obtained. 

 The following account of the lobster will serve quite as well 

 for the craw-fish, which abounds in the rivers and streams 

 of the Middle and Western States. 



The body of the lobster consists of segments {somites, 

 arthromeres), which in the abdomen are seen to form a com- 

 plete ring, bearing a pair of jointed appendages, which are 

 inserted between the sternum and tergum, the pleurum not 

 being well marked in the abdominal segments. The abdo- 

 men consists of seven segments. One of these segments 

 (Fig 226 £)') should be separated from the others by the stu- 

 dent, in order to observe the mode of insertion of the leg's. 

 Bach segment bears but a single pair of appendHges, and it 



