324 



THE CRAYFISH AND THE ARTHROPODA 



in correlation with the function of this part of the body in carrying 

 the eggs and embryos attached to the swimmerets. 



Other structures that are really external are the gills, and the 

 gill-cavity, which is formed by the ventral and posterior extension 

 of the thoracic skeleton as the carapace (Fig. 156). The skeleton 

 extends from the dorso-lateral region, like a man's coat if it were 

 sewed to his body along a line a few inches on either side of his 



Fig. 156. — Transverse section through thoracic region of crayfish. 



a, artery to appendage; o.&., afferent branchial blood vessel; c, carapace; e.b., efferent 

 branchial blood vessel; ff.c, gill cavity; ff./., gill filaments; /i, heart; to, muscle; n.c, nerve 

 cord; p. c, pericardial sinus; s, skeleton; u.o., sternal artery. The arrows show direction of 

 blood flow. (Redrawn from Lang, "Comparative Anatomy," copyright, 1891, by Macmil- 

 lan and Co., Ltd., printed by permission.) 



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backbone and widely opened on the ventral side where paired 

 appendages might be located. 



The skeleton, which clothes the entire outer surface of the cray- 

 fish, is an exoskeleton that envelops even the smallest external 

 parts and extends as a delicate lining for some distance into the 

 digestive tract at the mouth and anus. It is composed chemically 

 of an organic substance, chitin, to which is added carbonate of lime 

 in the thicker portions. The joints of the appendages and those 



