THE THEORY OF THE SKELETON. 161 
simply the interspaces left between the inferior or 
posterior wall of the prolongation of the carapace and 
the originally exposed external faces of these regions of 
the cephalic integument. 
Fourteen somites having thus been distinguished in 
the cephalothorax, and six being obvious in the abdomen, 
it is clear that there is a somite for every pair of append- 
ages. And, if we suppose the carapace divided into 
segments answering to these sterna, the whole body will ° 
be made up of twenty somites, each having a pair of 
appendages. As the carapace, however, is not actually 
divided into terga in correspondence with the sterna 
which it covers, all we can safely conclude from the 
anatomical facts is that it represents the tergal region of 
the somites, not that it is formed by the coalescence of 
primarily distinct terga. In the head, and in the greater 
part of the thorax, the somites are, as it were, run 
together, but the last thoracic somite is partly free and 
to a slight extent moveable, while the abdominal somites 
are all free, and moveably articulated together. At the 
anterior end of the body, and, apparently, from the an- 
tennary somite, the tergal region gives rise to the 
rostrum, which projects between and beyond the eyes. 
At the opposite extremity, the telson is a corresponding 
median outgrowth of the last somite, which has become 
moveably articulated therewith. The narrowing of the 
sternal moieties of the anterior thoracic somites, to- 
XN 
