266 



ZOOLOGY. 



Artienlata of Cnvier as a heterogeneous assemblage of 

 f onns embracing at least three branches of the animal king- 

 dom, namely, the Vermes, 

 Tv.nicata, and Arthropo- 

 da. 



The Arthropoda are di- 

 vided into six well-de- 

 fined classes, i. e., the 

 Crustacea with two body- 

 regions, the head-thorax 

 and abdomen (Fig. 220), 

 and breathing through 



fl, cephalo-thorax ; 6, abdomen. 



PalamwneUs vulgarif. 



the body-walls or by external gills; the Podostomata, which 

 are marine and breathe by gills, while the remaining four 

 classes breathe by internal air-tubes and live on land. These 

 are the Malacopoda, ilyriopoda, Arachn ida, and Inseda. 



Class L — Crustacea {^Water-fleas^ Shrimps, Lobsters, 

 and Crais). 



General Chara,cters 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 221 D') should be separated from the others by the stu- 

 dent, ta order to observe the mode of insertion of the legs. 

 Each segment bears but a single pair of appendages, and it 



