128 THE CRAYFISH 



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 

 movable. 



2. The abdomen is composed of seven movable segments, 

 united by peg and socket joints, which allow of 

 movement in the 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 seventh abdominal segment or telson is much 



flattened dorso-ventrally, and is devoid of pleura 

 and of appendages : it is divided transversely by 

 an imperfect hinge, and its ventral surface, which 

 is only slightly calcified, is perforated by the 

 longitudinal slit-like anus. 



C. The Appendages. 



These are all reducible to a common type composed of the 

 three parts, protopodite, exopodite, and endopodite, already 

 seen in the appendage of the typical segment, together with 

 an epipodite, which may be a simple plate or may be modified 

 to form a gill. From such a typical form all the appendages 

 of the crayfish can be derived by variations in form and 

 relative size of the various parts, or by total suppression of 

 one or more parts. 



