XIII. CRAYFISH, LOBSTERS, SPIDERS, AND 

 INSECTS 



Branch XI. — Arthropoda {arthron, joint; pous (pod) foot) 



Like the earthworms and leeches, the members of this 

 branch have segmented bodies. In addition to this, many 

 of these segments bear appendages of various kinds that, in 

 turn, are segmented; for example, the legs of insects. Such 

 appendages mark a decided advance over the worms. The 

 branch is very naturally divided into five classes, only four 

 oi which we shall discuss here. These four classes are 

 represented by the lobsters, spiders, centipeds, and in- 

 sects respectively. 



An Example of the Branch — the Crayfish 



The form and divisions of the body. — The crayfish has 

 a long, rather thin body, convex above but concave on the 

 ventral surface. It is covered, externally, by a hard, cal- 

 careous crust that protects the organs and furnishes places 

 of attachment for the muscles. The body is divided into 

 two regions : the anterior region, or cephalothorax, and the 

 posterior region, or abdomen (,Fig. 68) . The cephalothorax 

 is made up of the head and thorax closely joined and is 

 covered above and on the sides with a hard, shieldlike 

 structure known as the carapace. A transverse groove, the 

 cervical suture, on the surface of the carapace, separates the 

 head from the thorax. The abdomen is plainly divided 



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