HOMARTJS AMERICANUS. 153 



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

 tance of this reaction ? 



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

 The metameres of the head and thorax, however, are immova- 

 bly fused to form a cephalo-thorax. This is covered dorsally by 

 a single piece, the carapace. 



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. 

 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. The first 

 pair is greatly modified and the second pair bears a special 

 portion. 1 



(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. 



