XIII. CRAYFISH, LOBSTERS, SPIDERS, AND 



INSECTS 



Branch XL — Arthrripddu {arthron, joint; pons (pod) foot) 



Like the earthworms and leeches, the members of this 

 branch have segmented bodies. In addition to this, many 

 of these segments l)eai' ajjix-ndages of various Ivinds that, in 

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

 appendages mark a deeiiled advance over the worms. The 

 branch is very nat;n-ally divided into five classes, only four 

 of which we shall discuss here. These four classes are 

 represented by the lobsters, spidei's, centipeds, and in- 

 sects respectively. 



An Ex.\mple of titi: Branch — the Cr.wfish 



The form and divisions of the body. — The cra3^fish has 

 a long, rather thin body, convex al)ove l)ut concave on the 

 ventral .surface. It is coveied, externally, b\' a hard, cal- 

 careous crust that ]:>i-otccts the organs and furnishes places 

 of attachment for the muscles. The body is divided into 

 two I'cgions: the antei'ior region, or ccjilmlotJiorax, anil the 

 posterior region, or (lixlmiicii Fig. ()S). Tlie ceiihalothoi-ax 

 is made up of \hv. head and tliorax closel)' jome<l and is 

 covered aliove and on tlie sides \\\\\\ a. hard, shieldlike 

 structure known as tlic (rinrparc. A ti-ans-\'erse groove, the 

 cerricd] sviiir<'.()\\ (lie surface of the cai'npace, separates the 

 head from the thorax. Tlic .abdomen is plainly divided 



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