PHYLUM ARTHROPOD A 219 



is shed at the next molt. Injury is frequent in nature, and 

 consequently the claws are rarely of equal size. An injured claw- 

 is autotomously broken off by the animal at a special joint where 

 there is but little exposed to heal over. 



Abdominal Appendages. — Most of the abdominal segments 

 bear small biramous appendages termed swimmerets, except 

 in the crabs (Fig. 103) which have a rudimentary abdomen. 

 The appendages of the sixth abdominal somite are enlarged 

 and with the telson from a powerful tail fin. In the males, 

 the first and second abdominal appendages are highly modified 

 as a copulatory organ for the transfer of sperm. In the females, 

 the swimmerets serve as a place for the attachment of the eggs 

 during development. 



C 



Fig. 103. — The rock crab, Cancer irroratus. A, adult male, about one-fourth 

 size; B, larva in zoea stage; C, larva in megalops stage. {After Rathhun) . 



Respiration. — Gills are borne on the basal joints of certain 

 of the maxillipeds and legs and on the body wall. These are 

 usually contained within a branchial chamber formed by the 

 overhanging lateral folds of the carapace. A process of the 

 second maxilla becomes modified as a gill bailer the action of 

 which pumps water through the gill chamber. The gill chamber 

 retains water, thus keeping the gills moist and permitting 

 decapods to remain out of water for some time without injury to 

 the gills. In some crabs, the branchial chamber has become 

 modified as a lung in correlation with terrestrial habits. 



The circulatory system is of the open type. The heart 

 lies within a pericardial sinus in the dorsal region of the thorax. 

 Five arteries arise from the heart. Three of these pass ante- 

 riorly to the organs of the head and thorax, one passes along the 

 median dorsal line of the abdomen, and the last is directed ven- 

 trally either from the posterior boundary of the heart or as a 



