In these the two membranes, external and internal, the union of which constitutes the cornea, present simultaneously the division into facets, each of which forms anteriorly an ocular compartment proper to it. These facets, always hexagonal in insects, are of various forms in the Crustacea : thus in the Astacus fluviatilis, the Penew, the Galathe, and the Scyllari, they are square (fig. 399), whilst the Paguri, the Phyllosoma, the Squillw, the Gebiw, the anassw, and the Crabs, have them hexagonal (fig. 400). The talline that succeeds them diately is of a conical form, and is followed by a vitreous humour , having the appearance of a gelati nous filament, adhering by its base to the optic nerve. Each of the columns thus formed is, more over, lodged within a pigrnentary cell, which likewise covers the bulb of the optic nerve. But the most remarkable circumstance is, that the large cavity within which the whole of these parallel columns, every one of which is in itself a perfect eye, are contained, is closed posteriorly by a membrane, which ap pears to be neither more nor less than the tegumentary membrane, pierced for the passage of the optic nerve ; so that the ocular chamber at large results from the sepa ration at a point of the two external layers of the general envelope.
The gelatinous or vitreous elongated pro cesses which succeed the conical crystallines have been looked upon by several anatomists as ramifications of the optic nerve ; but we do not imagine that they are so in reality. In the Lobster, for instance, we have even seen the surface of the bulb isolated from the masses in question, divided into compartment:.
corresponding to those of the cornea itself, and lined with a layer of pigmentum perfectly distinct.
The most remarkable modification of facetted eyes consists in the presence of a kind of sup plementary lens, of a circular shape and set within the cornea in front of each proper crys talline lens (fig. 402). These small lenticular bodies exist independently, and are perfectly distinct from the small corneal facets. In some cases they might be mistaken (in the Idotew, for example, where they may be perceived singly, and with their distinct circular forms), and the incau tious observer led to conclude that the cor neal facets are merely these lenticular bodies so much enlarged that their hexagonal or square forms result from their agglomeration in a point; but there are Crustacea, such as the Callianassw, in which these two elements of the external cornea may be perfectly dis tinguished, the lenticular body being of insig nificant dimensions and occupying the centre of the corneal facet only (fig.402). In general,
however, the diameter of the lenticular body is equal to that of the corneal facet itself, so that their edges blend. Farther, the lenticular bodies are most commonly evolved in the sub stance of the cornea; but there are cases in .which, under favourable circumstances, they may be detached from it.
Although the existence of these different modifications must not be understood as being exclusive, inasmuch as there are certain Crus tacea which exhibit more than one of them at the same time, for instance, stemmata and compound eyes, the latter only are the species of visual organ encountered in the great ma jority of cases. Their general number is two ; but these are occasionally united, so as to form a single mass, and make the animal appear at first sight as if it had but a single eye. This peculiarity of organization can even be followed in the Daphnim, in the embryo of which the eyes are first seen isolated; with the progress of the development, however, they are observed gradually to approach each other, and finally to become united. Stemmata are always immoveable and sessile; the com pound eyes with smooth cornew, however, although in the majority of cases they present the same disposition, now and then occur moveable : sometimes they are supported by a pedicle, moveable in like manner, and pro vided with special muscles. The eyes with facets present the same modifications, and even supply important charac ters in classifying these animals : thus in the Edriophthalmia the eyes are always immoveable and sessile, (fig. 4030 whilst in the Decapo da and the Stomapoda (jig. 404) they are sup ported upon moveable stems of very various lengths, and which every consideration leads us to view as the limbs or appendages of the first cephalic ring. It some times even happens (fig. 404) that in these animals, between the outer edge of the cara pace and the base of the antennw, there occurs a furrow or cavity within which the eyes may be withdrawn or laid flat, so as to be out of the way of injury; this groove or cavity is generally spoken of under the name of the orbit.
§ 3. _Apparatus of Nutrition.
In the study of this apparatus we shall have to consider successively the organs of digestion, of circulation, of respiration, and of secretion.
A. _Apparatus y. digestion. — The organs concerned in the digestion of the food among the Crustacea may be divided into three orders, according to the functions they fulfil, to wit, 1st, the apparatus for the prehension and mastication of the food; 2nd, the alimen tary canal ; 3rd, the various secreting organs associated with the intestine.