144 THE CRAYFISH. 



is composed of five segments. In it are to be 

 noticed the following structures : 



i. The rostrum is a median pointed prolongation 

 of the carapace, extending forwards from its 

 anterior end. 



ii. The eyes are paired and mounted on movable 

 stalks, one at each side of the rostrum. 



iii. The mouth is far back on the ventral surface of 

 the head, and almost hidden by the jaws. It 

 is bounded in front by a fleshy protuberance, 

 the labrum. 



b. The thorax, or part behind the cervical groove, is 

 composed of eight segments. The following 

 points are to be noticed in it : 



i. The thoracic portion of the carapace is divided 

 by two longitudinal grooves about half an 

 inch apart ; the branchio-cardiac grooves, 

 into a median dorsal portion covering the 

 heart, and two large plates, the branchio- 

 stegites, which extend down at the sides of 

 the thorax and cover the gills. 



ii. The sterna of the thoracic segments are all fused 

 together, except the hindmost, which is mov- 

 able. 



2. The abdomen is composed of seven movable segments, 

 united by peg-and-socket joints, which allow of 

 movements in a vertical plane only, the limit of 

 extension possible being nearly a straight line. 



a. The first abdominal segment is smaller than the 



succeeding ones, and is devoid of pleura - 



b. The segments from the second to the sixth are 



essentially like the typical segment already 

 examined. 



c. The last abdominal region, or telson, consists of 



four pieces . (I) the segment proper traversed 



