570 



SHELL. 



tremely fine lines, passing in a direction 

 parallel to the surface of the shell and to each 

 other. The number of these is usually from 

 six to fifteen ; but they sometimes amount to 

 as many as thirty, or even sixty, their number 

 not being in relation either to the thickness of 

 the pigmentary layer, nor to the size of the 

 species observed ; but appearing to augment 

 with age. The cellular layer is that in which 

 the colouring matter of the shell is solely con- 

 tained ; but it does not always contain pig- 

 ment, its structure being precisely the same 

 on the white under-surface of the crab as on 

 the reddest portion of its carapace. When 

 examined with a low magnifying power, it pre- 

 sents an areolar aspect; but when a suffi- 

 ciently thin section is viewed by transmitted 

 light with a high magnifying power, the cha- 

 racter of the net-work, and of the dark spaces 

 it encloses, becomes at once apparent. It is 



Fig. 425. 



Cells of pigmentary layer of shell of Crab ; a, papil- 

 lary elevation of subjacent layer. Magnified 400 

 diameters. 



then obvious that the nearly colourless poly- 

 gonal reticulations are the thickened ivalls of 

 cells, each of them being divided by a distinct 

 line, which marks the junction of the conti- 

 guous boundaries ; whilst the dark spaces or 

 areolas are the cavities of the cells, filled with 

 colouring matter, or with some other semi- 

 opaque substance. This cellular layer is not 

 uniformly disposed over the entire surface of 

 the crab-shell ; for the calcareous layer beneath 

 rises up through it in little papillary eleva- 

 tions (fig. 425. ), to the summit of which the 

 epidermis adheres. It is from the deficiency 

 of the pigmentary layer at these points, that 

 the shell derives its minutely speckled appear- 

 ance. 



The internal layer is that which constitutes 

 by fur the thickest part of the shell of the 

 crab, and which must be regarded as its fun- 

 damental or essential element, since (according 

 to M. Lavalle) it is never wanting in the 

 Decapod Crustacea, whilst other layers are 

 sometimes deficient. It is in this internal 

 layer, that the calcareous matter is chiefly de- 

 posited ; but even after this has been re- 

 moved, a very distinct animal basis is left, 



possessing considerable firmness, and closely 

 resembling that which is left after the decal- 

 cification of dentine. When a thin section 

 of it is made parallel to its surface, and sub- 

 jected to a high magnifying power, it is seen 

 to be composed of an apparently homogeneous 

 substance, studded with minute points, each 

 surrounded by a clear space, which correspond 

 with those seen in a section of dentine cut at 

 right angles to the course of its tubuli, and 

 which would seem to possess the same essen- 

 tial character with them. A thin section of 

 the shell taken in the opposite direction (i. e. 

 from surface to surface) leaves no doubt, 

 when examined with a sufficient magnifying 

 power, of the nature of these markings ; for 

 they are then clearly seen to be the orifices 

 of tubuli, which pass with great regularity 

 from one surface of the shell to the other, 

 lying nearly parallel to each other, and having 

 their usually straight course interrupted at 

 tolerably regular intervals by minute sinuosi- 



Fig. 426. 



Portion of transverse section from claw oj Crab. 

 Magnified 400 diameters. 



ties resembling the " secondary curvatures " 

 described by Prof. Owen in the dentinal 

 tubuli. These sinuosities correspond with 

 bands which are seen to traverse the section, 

 running parallel to the surfaces of the shell ; 

 and they appear, like those of dentine, to 

 indicate the successive stages of calcification 

 of the animal basis. This structure is par- 

 ticularly well seen in the black extremities of 

 the claws of the common crab, in which the 

 intertubular substance is quite transparent in 

 a thin section, and of which the hardness and 

 density are as great as in many varieties of 

 dentine ; and as the tubuli are seen, in a 

 transverse section of the claw, to radiate from 

 the central cavity towards the surface, the 

 resemblance to a section of a tooth is alto- 

 gether so close, as quite to deceive an ob- 

 server unacquainted with the substance he is 

 examining. The same structure exists, how- 

 ever, in the remainder of the shell ; but from 

 some difference in its molecular constitution, 

 the intertubular substance has a less dense 

 and tenacious character, and has an opaque 

 chalky aspect, which renders even a very thin 



