ARTHROPODA — CRUSTACEA 



277 



or less lime carbonate. It is segmented because it is thickened 

 with lime carbonate in those regions not subject to bending, but 

 remains thin and hinge-like in the intermediate spaces. 



The carapace arises as a fold of the skin from the posterior 

 margin of the head region ; dorsally it coalesces with the body 

 segments, but at the sides it is free, forming the gill-covers (Fig. 

 123, g.c). As these covers are unattached below they permit the 

 free entrance of water to the gills within, which lie between the 

 sides of the carapace and the walls of the thorax proper. 



Fig. 124. — A young horseshoe crab, Limulus polyphemiis, from the Massachusetts 

 coast, molting, abd., abdomen; ceph., cephalothorax ; e., a compound eye; 

 e'., a simple eye ; I., legs ; n., newly molted portion ; 0., the edges of the old shell ; 

 /., telson. 



Since the soft parts of the body are completely incased in 

 resistant chitin, growth can take place only by a process of 

 molting, — of shedding the chitinous covering periodically (see 

 Fig. 124). This is thrown off as a distinct whole. Not only 

 is every part of the exterior shed, but also the lining of the 

 oesophagus, stomach, and all of the intestine except the middle 

 portion, though some internally shed chitin, however, as that 

 of the stomach, does not leave the stomach, but is redigested. 

 This molt, an exact copy of the exterior of the living animal, 

 is shed about once a year during adult life, but oftener during 

 the rapid growth of youth. 



