WILLIAMSON, ON SHELLS OF CRUSTACEA. .3/ 



unable to determine, but I am disposed to believe that 

 they do. 



We often find a third modification of the areola?, in which 

 the centre of each is occupied by a dark radiating spot re- 

 sembling a stellate pigment -cell ; and I have occasionally 

 seen in the common crab, a form identical with that seen by 

 Professor Quekett in a species of Portumnus, and which he de- 

 scribes as consisting of " hexagonal cells having thick walls" 

 (' Lectures on Histology/ vol. ii, p. 393). No doubt exists 

 on my own mind that all these are mere varieties dependent 

 on slight modifications of the calcific process. 



PI. Ill, fig. 1, presents a diagram of the arrangement of the 

 layers in the shell of the crab; a being the pellicle; b, the areo- 

 latedpart with its small tubules; d,the calcified corium with its 

 cylindrical pillars and vertical tubuli; and/, the uncalcified 

 corium. Recalling what I have said respecting the tendency of 

 the tubuli in the areolated layers to assume directions per- 

 pendicular to the plane of the laminae which they penetrate, 

 a glance at the diagram will explain the " radiated cells" of 

 Professor Quekett, which he describes as surrounding the 

 pillars of the corium ( loc. cit., vol. ii, fig. 256). In any 

 horizontal section of these structures, the tubuli at a distance 

 from the pillars would be intersected at right angles to their 

 direction, whilst near the pillars, owing to the change in the 

 plane of the component lamina?, the tubules would be inter- 

 sected nearly in the direction of their length. Hence the 

 appearance of radiating lines in which Professor Quekett 

 could detect no cell-wall, and which he found so difficult to 

 reconcile with his idea of cellular structure. 



The upper laminae of the corium are thicker than the 

 lower ones ; and the undulations of the penetrating tubuli, 

 so characteristic of crustacean structures, appear to be de- 

 finitely related to the laminae through which they pass. 

 Towards the upper part of each pillar, the tubuli bend 

 outwards from the same reason that those of the contiguous 

 areolae are deflected from a perpendicular line, viz., the ten- 

 dency to penetrate the laminae at right angles to their plane. 

 I have not detected any branching forms amongst these 

 tubuli, though Professor Queckett thinks that he has observed 

 such. 



Beneath the calcified corium is the thin im-calcified layer of 

 the same tissue, which would ultimately have become calcified, 

 as fresh laminae were applied to its inner surface. When 

 detached from the calcified shell, which is readily done, it 

 presents numerous granular specks, which obviously corre- 

 spond with the tubuli of the calcified tissue. I have found 



