TEGUMENTARY ORGANS. 



487 



a minute polygonal areolation is observable at 

 times upon tlie most superficial " epidermic " 

 layer. I do not know from what cause this 



Fig. 312. 



tjrrg, 

 i 



L '"^: A 



A to E, from tlie crab ; F, G, from the shrimp. 



proceeds, but the areolae are certainly not 

 cells. 



Such is the structure of the soft inter- 

 articular integument, in which I could find 

 no calcareous matter. If it be traced into 

 the hard calcined shell, the only alteration 

 observable is, that a dark calcareous deposit 

 takes place, the earthy matter being infil- 

 trated, as it were, through the laminae. The 

 deposit affects, however, only the middle 

 layers, the extreme outermost being left as 

 the "horny epiderm;" while a variable number 

 of the inner laminae 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. 



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 Lavalle 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 shrimp, M. Lavalle is 

 right. I believe, in fact, that tlie 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 laminas 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 laminae, 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 areae, 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 (_/%. 312. D) 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, il 

 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 

 laminae. 



I believe, then, that the " cellular" laj'er, 

 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 ami 

 striated, and not tubular. The former owe 



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