23G THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



pleura of the sixth somite are narrow, and their posterior 

 edges are concave. 



The pits and setse of the. cuticle which clothes the 

 tergal surfaces of the abdominal somites are so few and 

 scattered, that the latter appear almost smooth. In the 

 telson, however, especially in its posterior division, the 

 markings are coarser and the setae more apparent. 



The telson (fig. 61, G) presents an anterior quadrate divi- 

 sion and a posterior half- oval part, the free curved edge 

 of which is beset with long setse, and is sometimes slightly- 

 notched in the middle. The posterior division is freely 

 movable upon the anterior, in consequence of the thin- 

 ness and pliability of the cuticle along a transverse line 

 which joins the postero-external angles of the anterior 

 division, each of which is produced into two strong spines, 

 of which the outer is the longer. The length of the pos- 

 terior division of the telson, measured from the middle 

 of the suture, is equal to, or but very little less than, 

 that of the anterior division. 



On the under side of the head, the basal joints of the 

 antennules are visible, internal to those of the antennse, 

 but the attachment of the latter is behind and below 

 that of the former (fig. 3, A). Behind these, and in 

 front of the mouth, the epistoma (fig. 39, A, II, III) 

 presents a broad area of a pentagonal form. The pos- 

 terior boundary of this area is formed by two thickened 

 transverse ridges, which meet on the middle line at a 

 very open angle, the upex of which is turned forwards. 



