27^ THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



not used as a swimming organ ; and the sixth somite 

 possesses no appendages whatever. The breadth of the 

 cephalothorax is often greater than its length, and there 

 is no prominent rostrum. In its place there is a trun- 

 cated process (fig. 72, B, r), wliich sends down a vertical 

 partition and divides from one another two cavities, in 

 which the swoUen basal joints of the small antennules {2) 

 are lodged. The outer boundary of each of these cavities 

 is formed by the basal part of the antenna (5), which is 

 firmly fixed to the edge of the carapace. There is no exo- 

 poditic scale ; and the free part of the antenna (5') is very 

 small. The convex corneal surface of the eye appears 

 outside the base of the antenna, lodged in a sort of orbit 

 {or), the inner margin of which is formed by the base of 

 the antenna, while the upper and outer boundaries are 

 constituted by the carapace. Thus, while in all the pre- 

 ceding forms, the eye is situated nearest the middle line, 

 and is most forward, while the antennule lies outside 

 and behind it, and the antenna comes next ; in the crab, 

 the antennule occupies the innermost place, the antenna 

 comes next, and the ej'e appears to be external to and 

 behind the other two. But there is no real change in 

 the attachments of the eye-stalks. For if the antennule 

 and the basal joint of the antenna are removed, it will be 

 seen that the base of the eye-staUc is attached, as in the 

 crayfish, close to the middle line, on the inner side, 

 and in front of the antennule. But it is very long and 

 extends outwards, behind the antennule and the antenna ; 



