394 TEGUMENTARY ORGANS 



integument {^fig. 312. E). Finally, if the decalcified scale include a 

 sufficient number of layers, it is easy, by altering the focus of the 

 microscope, to trace the areolation inwards, until it becomes gradually 

 fainter, and disappears, passing into the ordinary dotted laminae. 



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 carapace) 

 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, 

 transparent, calcareous deposit, which allows the previous dotted 

 structure of the laminae to be perfectly obvious. In some parts and 

 in the superficial layers, this deposit is structureless and homogeneous 

 {fig. 312. G, a), but in other parts the youngest layer presents very 

 delicate polygonal meshes, whose areae were about xwo i"- i" 

 diameter {^fig. 312. F, a). Decalcification completely destroys this 

 appearance ; so that I imagine it to be caused merely by the mode in 

 which the primary deposit in the membrane takes place, the areolae 

 becoming almost immediately fused together by further deposit. 



Through these homogeneous hardened outer layers thus con- 

 stituted, there are dispersed more opaque spots {^fig. 312. F, G, b), 

 more or less rounded in their outline, and varying in diameter from 

 1 in or less, to ten times that size. The smallest of these bodies 



5 ' 



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 gradually 

 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 cal- 

 careous 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 development may be so plainly demonstrated, has a most 

 important application, not only to the mode of formation of 



