THE ENDOPHPvAGMAL 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 /it(viatilis.— 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 reahty, 

 however, the curious pillars and bulkheads which enter 

 into the composition of the endophragmal S3'stem are all 



* There are some sin g-ular marine Crustacea, the SqnUlidce, in which 

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

 while the rostrum is articulated with the tergum of the antennary 

 somite. 



