88 J. SLADE ON THE MICROSCOPIC STRUCTURE 



by Quekett, in liis Histology, by Huxley, in the Cyclo. Anat. 

 and Physiology, and by Professor Williamson, in the " Transactions 

 of the Royal Microscopical Society." 



The four layers of which the shell is composed are well seen in a 

 section of a piece of the carapace, taken at right angles to the surface. 

 It is then found that the external and internal layers are entirely struc- 

 tureless — the external probably forming a mere protective covering 

 to the middle layers, — the internal, probably, in the course of growth 

 becomes converted into the adjoining layer. These layers are mem- 

 branous and structureless, and do not contain calcareous matter. 

 The inner layers are known as the areolar layer and the corium. 

 The areolar layer is the uppermost of the two, and lies immediately 

 beneath the external layer. It is termed cellular by Dr. Carpenter, 

 but Huxley and Williamson have shown that it is not cellular ; 

 but upon its surface, immediately in contact with the external 

 layer, it is seen to be areolated. The vertical section shows it to be 

 composed of thin laminte. Beneath this areolar layer lies the 

 calcified corium, — this is also composed of fine laminae. These 

 laminre, at first, lie flat and horizontal with the inner layer, but 

 gradually, at numerous points, they rise, and at last become so much 

 inflected that they ascend like flat-topped cylindrical pillars, pene- 

 trating the areolar layer, and reaching the external pellicle, and 

 there form the white spots seen on the surface of the crab's shell. 

 The tubules composing the tubulated structure traverse the areolated 

 layer and corium at right angles to the laminse, of which they are 

 composed ; and where these are inflected the tubules bend to suit 

 the altered condition of things. The tubules of the areolated layer 

 can be seen easily, and are large compared with those of the corium, 

 which are very fine, crowded and confused, and not easily traced in 

 balsam mounted specimens. 



Flat sections of the shell, those made parallel to the surface, 

 present very different appearances according to the position at 

 which they are taken. 



In a section of the claw it is seen that the corium does not rise 

 in cylindrical pillars, and the tubules are more distinctly seen and 

 more closely resemble dentine. 



The structure of the shell of the crab is that which is always 

 found in all the most highly organised forms of the Decapodous 

 Crustacea. And in the group Brachyura all the layers previously 

 described are present, and the variations consist in the different 



