TEGUMENTARY ORGANS 393 



variable number of the inner laminje 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 structureless as where they constitute the deep layer of the inter- 

 articular membranes. 



The structure of the calcified layer has been carefully described by 

 Dr. Garpenter, 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 the tubular 

 structure is produced by the horizontal lamination 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 lamina; 

 composed 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 ares, 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 manner in which the calcareous matter 

 is deposited. We have seen, in fact, that there are no such appearances 

 in the deep uncalcified 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 " ceils," but in their place we observe clear polygonal spaces in 

 the membrane {fig. 312. D) which present the same dots (section of 

 tubules) as those which exist in the simply laminated portion of the 



