The epimera of the head are, for the most part, very narrow; but those of the antennulary somite are broad plates (fig. 40, _epm._), which constitute the posterior {157} wall of the orbits. I am inclined to think that a transverse 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.[11] [11] There are some singular marine crustacea, the _Squillidæ_, 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. 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. [Illustration: FIG. 41.—_Astacus fluviatilis._—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 affording attachment to muscles, and serving to protect important 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 {158} mere infoldings of the cuticle, or _apodemes_; and, as such, they are shed along with the other cuticular structures during the process of ecdysis. Without entering into unnecessary details, the general principle of the construction of the endophragmal skeleton may be stated as follows. Four apodemes are developed between every two somites, and as every apodeme is a fold of the cuticle, it follows that the anterior wall of each belongs to the somite in front, and the posterior wall to the somite behind. All four apodemes lie in the ventral half of the somite and form a single transverse series; consequently there are two nearer the middle line, which are termed the _endosternites_, and two further off, which are the _endopleurites_. The former lie at the inner, and the latter at the outer ends of the partitions or _arthrophragms_ (fig. 39, A, _a, a′_, fig. 42, _aph_), between the articular cavities for the basal joints of the limbs, and they spring partly from the latter and partly from the sternum and the epimera respectively.