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occupied only by soft or imperfectly calcified cuticle, which, on each side of the hinder part of the mouth, passes into one of the lobes of the metastoma (mt). At the base of each of these lobes there is a calcified plate, united by an oblique suture with another, which occupies the whole length of the lobe and gives it firmness. The soft narrow lip which constitutes the lateral boundary of the oral aperture, and lies between it and the mandible, passes, in front, into the posterior face of the labrum (lb).

In front of the mouth, the sternal region which appertains, in part, to the antennæ, and, in part, to the mandibles, is obvious as a broad plate (III), termed the epistoma. The middle third of the posterior edge of the epistoma gives rise to a thickened transverse ridge, with rounded ends, slightly excavated behind, and is then continued into the labrum (b), which is strengthened by three pairs of calcifications, arranged in a longitudinal series. The sides of the front edge of the epistoma are excavated, and bound the articular cavities for the basal joints of the antennæ (3); but, in the middle line, the epistoma is continued forwards into a spear-head shaped process (figs. 39 and 40, II), to which the posterior end of the antennulary sternum contributes. The antennulary sternum is very narrow, and its anterior or upper end runs into a small but distinct conical median spine (fig. 40, t.). Upon this follows an uncalcified plate, bent into the form of a half cylinder (I), which lies between the inner ends of

the eye-stalks and is united with adjacent parts only by flexible cuticle, so that it is freely movable. This represents the whole of the sternal region, and probably more, of the ophthalmic somite.

The sterna of fourteen somites are thus identifiable in the cephalothorax. The corresponding epimera are

FIG. 40.-Astacus fluviatilis.—The ophthalmic and antennulary somites (× 3). I, ophthalmic, and II, antennulary sternum; 1, articular surface for eyestalk; 2, for antennule; epm, epimeral plate; pep, procephalic process; r, base of rostrum ; t, tubercle.

represented, in the thorax, by the thin inner walls of the branchial chamber; the pleura, by the branchiostegites; and the terga, by so much of the median region of the carapace as lies behind the cervical groove. That part of the carapace which is situated in front of this groove occupies the place of the terga of the head; while the low ridge, skirting the oral and præ-oral region, in which it terminates laterally, represents the pleura of the cephalic somites.

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

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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.*

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 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

* 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.

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.

The endosternite (fig. 42, ens.) ascends vertically, with a slight inclination forwards, and its summit narrows and assumes the form of a pillar, with a flat, transversely elongated capital. The inner prolongation of the capital is called the mesophragm (mph.), the outer the paraphragm (pph.). The mesophragms of the two endosternites of a somite usually unite by a median suture, and thus form a complete arch over the sternal canal (s.c.), which lies between the endosternites.

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The endopleurites (en.pl.) are also vertical plates, but they are relatively shorter, and their inner angles give off two nearly horizontal processes, one of which passes obliquely forwards (fig. 39, B, h, fig. 42, h.p.) and unites with the paraphragm of the endosternite of the somite in front, while the other, passing obliquely backwards (fig. 39, h'), becomes similarly connected with the endosternite of the somite behind.

FIG. 42.—Astacus fluviatilis.-A segment of the endophragmal system (x3). aph, arthrophragm; arth, arthrodial or articular cavity; cxp, coxopodite of the ambulatory leg; empl, endopleurite; as, endosternite; epm, epimeron; hp. horizontal process of endopleurite; mph, mesophragm; pph, paraphragm; 8. sternum of somite; &c, sternal canal.

The endopleurites of the last thoracic somite are radimentary, and its endosternites are small. On the other hand, the mesophragmal processes of the endosternites of the two posterior somites of the head (fig. 39, B, c.ap), by which the endophragmal system terminates in front, are particularly strong and closely united together. They thus, with their endopleurites, form a solid partition between the stomach, which lies upon them, and the mass of