274 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), which sends down a vertical 
partition, and divides from one another two cavities, in 
which the swollen 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 (3), which is 
firmly fixed to the edge of the carapace. There is no exo- 
poditic scale; and the free part of the antenna (3’) 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 eye 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-stalk 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; 
