HOMARUS AMERICANUS. 157 



will respond, but sometimes not promptly. What is the im- 

 portance of this reaction? 



External Anatomy. — As in Nereis, the body is segmented. 

 The five segments of the head and the eight segments of the 

 thorax, however, are immovably fused to form a cephalo- 

 thorax. This is covered dorsally by a single piece, the cara- 

 pace. 



1 . Note, on the carapace, the cervical groove between the head 

 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. 

 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 pleiiroii), 

 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. 



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, antenrwe, 

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

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

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

 pleopods are modified to form copulatory organs. The first 

 pair is greatly modified and the second pair bears a special 

 portion.^ 



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

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

 1 The crayfish has the first two pairs, both greatly modified. 



