138 ARTHROPODA. 



and thorax, and the beak or rostrum forming an anterior spine. 

 The ventro-lateral edge of the carapace is not attached. A flat 

 object thrust between it and the body passes into the gill cham- 

 ber. This free plate of the carapace is called the gill-cover. 

 Do you know why the edge of the carapace is free? Notice the 

 hair-like spines along its free border. What purpose do these 

 serve ? 



2. The abdomen is composed of seven movable segments, 

 each bearing a pair of jointed appendages except the last, which 

 is sometimes not considered a true segment and is called the 

 telson. Each abdominal segment consists of a dorsal piece, the 

 tergum, which is continued as a free plate laterally (the pleurori), 

 and of a ventral piece, the sternum. Move the abdominal seg- 

 ments and see where they are hinged. How are the terga and 

 sterna arranged to allow free movement? In the thorax the 

 sterna, though fused, can be distinguished. There are eight 

 segments in the thorax. 



3. Appendages. Aside from the stalked eyes, whose homol- 

 ogy with true appendages is doubtful, there are nineteen pairs. 

 These are, counting from before backward: antennules, antennae, 

 six pairs of mouth appendages, five pairs of walking legs (pereio- 

 pods), of which the first are the claws or chelce, and six pairs of 

 swimmerets (pleopods). In the male, the first two pairs of 

 pleopods are modified to form copulatory organs. 



(a) Turn one of the fifth pair of pleopods forward and exam- 

 ine its posterior aspect. It consists of a basal piece, the proto- 

 pod, a lateral branch, the exopod, and a median branch, the 

 endopod. This branched type of appendage is designated as 

 biramous. What is its use? Compare with this the modified 

 sixth pair of pleopods, called the uropods. 



Make a drawing of one of the -fifth pleopods. 



(6) In front of the chelae will be seen the sixth pair of mouth 

 appendages, the third maxillipeds. Remove that of the right 

 side and compare it with the fifth pleopod. In addition to the 

 protopod, exopod, and endopod, it bears a long blade, the epipod, 

 which extended into the gill chamber. The protopod is com- 



