§ 1. Of the skin or tegumentary skekton, and qf the organs qf loco/notion.
In the definition which has been given of the Crustacea, one of the most important cha racters was derived from the nature and dispo sition of their tegumentary system. And it is from this point that we shall start in laying before our readers a detailed account of the peculiarities of organization presented by this class of animals. By pursuing this course all the subsequent parts of the present article will appear clearer, the disposition of the internal organs, their forms, their mutual relations, &c. being in a great number of instances readily explicable by the various modes of confor mation of the modified skin, which in this class performs the important office of the in ternal skeleton among the Vertebrata.
In some Crustacea the skin always con tinues soft, but in the greater number it presents a great degree of solidity, and forms a solid casing, within which are included the whole of the soft parts. This difference in the condition of the tegumentary envelope is generally found to coincide with the pre sence or absence of particular organs for the purposes of respiration ; and in fact it is easy to -understand that in those species in which this important function is performed by the surface of the body at large, the integument required to be membranous, whilst in those in which the covering is of stony hardness, a con dition which renders it incompetent to expose the blood to the contact of the atmospheric air dissolved in water, respiration can only be performed by the medium of organs especially contrived and set apart for the purpose.
When the tegumentary envelope of the Crus tacea is studied among the more elevated indi viduals of the class, it is found to possess a somewhat complex structure ; parts may be distinguished in it comparable to those which are known to constitute the integument of the Vertebrata. Among the Brachyura, for in stance, the integument consists of a corium and an epidermis with a pigmentary matter of a peculiar nature destined to communicate to the latter membrane the various colours with which it is ornamented.
The corium or dermis, as among the Verte brata, is a thick, spongy, and very vascular mem brane; on its inner surface it is intimately con nected with a kind of serous membrane, which lines the parietes of the cavities in the Crus tacea in the same manner as the serous mem branes line the internal cavities among the Ver tebrata ; these two membranes, divided in the latter order by the interposition of muscular and bony layers, which cover and protect the great cavities, become closely united vvhen these layers disappear, as they do in the Crus tacea in consequence of the important changes that take place in the conformation of the ap paratus of locomotion.
The corium, again, among the Crustacea, is completely covered on its outer surface by a membranous envelope unfurnished with blood vessels, and which must be held in all respects as analogous to the epidermis_ of the higher animals. It is never found in the properly membranous state, save at the time of the Crus tacea casting their shell ; at this period it is interposed between the corium and the solid covering, ready to be cast off, and has the appearance of a pretty dense and consistent membrane, in spite of its thinness. It forms, as among animals higher in the scale, a kind of inorganic lamina, applied to the surface of the corium, from which it is an exudation. After the fall of the old shell, it becomes thicker and very considerably firmer, owing to the deposi tion or penetration of calcareous molecules within its substance, as well as by the addition of new layers to its inner surface. The degree of hardness finally acquired, however, and the amount of calcareous matter deposited within it, vary considerably; in many members of the class it remains semi-corneous, in a condition very similar to that of the integuments of in sects, with which, moreover, it corresponds very closely in point of chemical composition; in the higher Crustaceans, again, its composi tion is very different : thus, whilst chitine in combination with albumen is the principal element in the tegumentary skeleton of some species, this substance scarcely occurs in the proportion of one or two-tenths in the carapace of the Decapods, which, on the contrary, con tains sixty and even eighty per cent. of phos phate and carbonate of lime, the latter sub stance particularly occurring in considerably larger proportions than the former.* With regard to the pigmentum, it is less a membrane or reticulation than an amorphous matter diffused through the outermost layer of the superficial membrane, being secreted like this by the corium. Alcohol, ether, the acids, and water at 2120 Fahr. change it to a red in the greater number of species ; but there are some species in which it may be exposed to the action of these different agents without under going any perceptible change.f The epidermic layer hardened in different degrees is the part which mainly constitutes the tegumentary skeleton of the Crustacea. In its nature it is obviously altogether different from that of the internal skeleton of the Verte brata ; still its functions are the same, and this physiological resemblance has led naturalists to speak of these two pieces of organic mecha nism, so dissimilar in their anatomical rela tions, under the common name of skeleton.