THE ENDOPHEAGMAL SYSTEM. 157 



wall of the orbits. I am inclined to think that a trans- 

 verse ridge, which unites these under the base of the 

 rostrum, represents the tergum of the antennulary somite, 

 and that the rostrum itself belongs to the next or 

 antennary somite.* 



The sharp convex ventral edge of the rostrum (fig. 41) 

 is produced into a single, or sometimes two divergent 

 spines, which descend, in front of the ophthalmic somite, 

 towards the conical tubercle mentioned above : it thus 

 gives rise to an imperfect partition between the orbits. 



Fig. 41. — Astacns flitviatilis. — The rostrum, seen from the left side. 



The internal face of the sternal wall of the whole of 

 the thorax and of the post-oral part of the head, presents 

 a complicated arrangement of hard parts, which is known 

 as the endophragmal system (figs. 39, B, 42, and 43), and 

 which performs the office of an internal skeleton by afford- 

 ing attachment to muscles, and serving to protect im- 

 portant viscera, while at the same time it ties the somites 

 together, and unites them into a solid whole. In reality, 

 however, the curious pillars and bulkheads which enter 

 into the composition of the endophragmal system are all 



* There are some singular marine Crustacea, the Sqinllidai, in which 

 both the ophthalmic and the antennary somites are free and movable, 

 while the rostrum is articulated with the tergum of the antennary 

 somite. 



