The integument thus described closely re sembles that of the larger Crustacea (vide infra), and I should have placed it with them, except for the very distinct statement of Mr. Newport with regard to the development of the integument in Meli5e. According to Mr. Newport's researches, the integument of the young Meltie is at first composed of polygonal nucleated cells, the largest of which is about „1,, of an inch in diameter. As the animal grows, the nuclei divide and subdivide by a process of fission, and the integument becomes composed of several layers. After awhile, the deeper of these undergo a fibrous meta morphosis, and constitute a fibro-cellular structure, which gives attachment internally to the muscles, while the external layers continue to grow, and to be reproduced as distinct cells.
If this were the mode of development which obtained in all Insecta we must con sider their chitinous integument to be pro duced by conversion of the previously ex isting cells of the ecdcron. However, Leydig's statements are equally decided, that the in tegument of Corethra presents no appearance of cellular origin, and the question may, there fore, for the present, probably be considered undecided.
The calcified integument of the Crustacea presents tbe same general structure as that of the other Annulosa, consisting of superposed chitinous, more or less fibrous lamellm, the outer of which are infiltrated with a calcareous deposit. In the small transparent Crustacea, as we have already seen, the integument is composed of structureless layers, developed by excretion on the surface of the ecderon, and even in the largest forms, the minute hairs, &c., present precisely the same ap pearance ; but in the thick integument of the Decapoda, certain layers of the shell have been described, not without considerable show of reason, as possessing a cellular organisation (Carpenter).
I have carefully examined the shell of the common crab in relation to this point, and the following are the results of my investigation.
It appeared to me in the first place, that, without seeking for a moulting crab, the structure of the integument in its uncalcified state might be readily ascertained by ex amining the soft membrane connecting the articulations of the limbs, which, as is well known, is continuous on either hand with the calcareous integument, and passes into it. In a section of this soft layer (fig. 312. A), I found from within, outward, 1. The enderon (a) composed of connective tissue, excavated by vascular channels, and containing nume rous aggregations of pink and yellow pigment, frequently disposed in a stellate form, or even forming anastomosing net-works along the ru dimentary' elastic fibres of the tissue. 2. The
surface of tbis (b) was constituted by a proto morphic layer, consisting of a homogeneous substance containing endoplasts (c), which sometimes adhered to the enderon, sometimes to the hard integument, when the latter was detached. 3. Superficial to this, was the chitinous layer of the integument (c) com posed of a number of laminm of great deli cacy, and not more than ,D1,5-[, of an inch apart. The deep laminm were much softer than the superficial, and the outermost lamina of all was hardest, and of a brownish colour, constituting the structureless epidermis of Carpenter and Lavalle.
In section, the deep laminm (u) presented only an indication of perpendicular fibrillation; but this became more marked superficially, the outer part of the section appearing closely striated. The deep laminm, when stripped off, presented no definite structure, but they readily fell into plaits ; while the superficial lamince ap peared dotted over. I sought in vain for any appearance of endoplasts in the deep layer, where, however, had they existed, they must have been readily detected ; and I therefore conclude, that the chitinous lamellm are formed from the subjacent ecderon, by a process of ex cretion. It should be remarked, however, that a minute polygonal areolation is observable at times upon the most superficial "epidermic " layer. I do not know from what cause this proceeds, but the areolm are certainly not cells.
Such is the structure of the soft inter articular integument, in which I could find no calcareous niatter. If it be traced into the hard calcified shell, the only alteration observable is, that a dark calcareous deposit takes place, the earthy matter being infil trated, as it were, through the laminm. The deposit affects, however, only the middle layers, the extreme outermost being left as the "horny epiderm;" while a variable number of the inner laminm remain as a more or less thick, soft coating upon the inner surface. These soft layers may be stripped off as a parchment-like membrane, with the muscles, and their relations to the enderon are then readily examined. They are here as struc tureless as where they constitute the deep layer of the inter-articular membranes.