The structure of the calcified layer has been carefully described by Dr. Carpenter, who showed, that in the crab and lobster they are traversed by tubules identical with those of dentine, and pointed out the error of LavaIle in regarding these as fibres. There can, I think, be no doubt, that in the crab and lobster, Dr. Carpenter's doctrine is correct ; but I am equally of opinion, that for other crustacea, such as the shrirnp, Lavalle is right. I believe, in fact, that the tubular structure is produced by the horizontal lami nation giving way, as the calcareous matter is deposited, to perpendicular fibrillation of the chitinous matrix, and that, eventually, the uncalcified fibrils disappear, and leave tubules in their place. That at least appears to be a natural conclusion from the fact, that the perpendicular fibrillation of the soft tissue becomes more and more marked externally ; and thus, by decalcifying the calcified shell, we obtain horizontal separable laminm com posed of short perpendicular fibres.
The colouring matter has always appeared to me to be generally diffused through the upper layer, and not to be confined to what Dr. Carpenter describes as the " cellular layer." The latter is a very thin stratum, made up of only a few of the superficial laminm, which I have found to be most readily observable by detaching with a sharp knife a very thin scale from the upper surface of the crab shell. It is composed, exactly as Dr. Carpenter has figured it, of regularly polygonal, often six sided arem, frequently presenting a darker radiating patch in the centre, and, at first sight, irresistibly suggesting a true cellular structure.
I believe, however, that it is in reality nothing of the kind, but that, like similar appearances in the molluscan shell, this is simply the result of the concretionary 'man ner in which the calcareous matter is de posited. We have seen, in fact, that there are no such appearances in the deep uncal cified layers, nor in the thin layers which invest the minute transparent appendages— considerations which appear to me to be in themselves decisive against the cellular nature of these bodies. In addition, decalcification brings to light no endoplasts in the " cells," but in their place we observe clear polygonal spaces in the membrane (fig. 312. n) which present the same dots (section of tubules)as those which exist in the simply laminated por tion of the integument (fig. 312. E). Finally, it the decalcified scale include a sufficient num ber of layers, it is easy, by altering the focus of the microscope, to trace the areolation in wards, until it becomes gradually fainter, and disappears, passing into the ordinary dotted I believe, then, that the " cellular" layer, results from a peculiar additional deposit of calcareous matter in the uppermost layers of the shell; and this view is strikingly confirmed by what may be observed in the shrimp. The
integument in this crustacean (e. g. the cara pace) has exactly the same general structure as that of the crab, consisting of hard upper and soft deep layers, which are dotted and striated, and not tubular. The former owe their hardness to a generally diffused, trans parent, calcareous deposit, which allows the previous dotted structure of the laminw to be perfectly obvious. In some parts and in the superficial layers, this deposit is struc tureless and homogeneous (fig. 312. G, a), but in other parts the youngest layer presents very delicate polygonal meshes, whose arem were about Tiko• in. in diameter (fig.312. F, a). De calcification completely destroys this appear ance ; so that I imagine it to be caused merely by the mode in which the primary deposit in the rnembrane takes place, the areolre becoming almost immediately fused together by further deposit.
Through these homogeneous hardened outer layers thus constituted, there are dispersed more opaque spots ( fig. 312. F, G, b), more or less rounded in their outline, and varying in diameter from ,413. in. or less, to ten times that size. The smallest of these bodies have exactly the appearance of cells (fig. 312. F, b), consisting of a dark centre, with a circular more transparent wall, and every variety of form may be observed between these and large masses, such as that figured (fig. 312. G, b), with a lobulated laminated circumference, and an irregular centre, composed of small masses like dentine globules. In the former the dots of the original tissue may be still seen ; but in the latter they are not traceable and seem to be obliterated. If dilute hydrochloric acid be added while the object is still under the microscope, however, these bodies are gra dually dissolved out with effervescence,and the structure of the place they occupied is found to be identical with that of the other portions of the integument. They are, therefore, nothing but concretions of calcareous matter, whose deposit has taken place in a peculiar form, quite independently of the primary structure of the part ; this form being, in the smaller concretions, most deceptively cell-like. It appears to me that this case, in which the assumption of structure without cell develop ment may be so plainly demonstrated, has a most important application, not only to the mode of formation of Crustacean and Mol luscan shells (vide infra), but to the develop ment of the teeth, strongly confirming, I think, the view which I have taken of that pro cess.