THE ENDOPHRAGMAL 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.—Astacus fuviatilis.—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 48), 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 Sgwillide, 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. 
