ON THE MICROSCOPIC STRUCTURE OF SHELLS. 
129 
more than thirty in a Lobster, and more than sixty in a Palinurus of ordinary 
size. This number, M. Lavalle remarks, is in relation neither with the 
thickness of the pigmentary layer, nor with the size of the species observed ; 
it appears to augment with age. I have not myself noticed these lines— 
probably becau-'e the pigmentary layer in the specimens 1 have examined 
was too thiu to exhibit them ; but they seem to be analogous to those shown 
i» fig?. 80 and 81 in the subjacent calcified layer. 
125. The internal or dermal layer is that which constitutes by far the 
thickest part of the shell of the Crab; ami must be regaialcd as its funda¬ 
ment^ or essential element, since (according to M, Lavalle) it is never 
wwitingin the Decapod Crustacea, whilst the other layers are sometimes de¬ 
ficient. It is in this internal layer that the calcareous matter is chiefly de- 
pttsited; but even after this has been removed, a very dUtiucl animal basis 
ii left, poMessing considerable substance, and analogous to that which is left; 
!^ier the decalcification of dentine. The lin<*s wli tcli are seen in a vertical sec¬ 
tion of the shell to be running parallel to the surface and to each other 
(fip* 60.81), closely correspond with those which are presentrtl by many 
^rieties of dentine, and seem to indicate the successive stages of the calci- 
ncation of the tests from without inwards. These line* are stated by M. 
lavalle to present themselves in the .shells of all the Crustacea which he has 
examined, with the exception of certain Auuraoura ; and in these exceptional 
canes the dermal layer presents the further peculiarity of distinct iridescent 
cdourg. This coloration, he adds remains after the removal of the calca¬ 
reous rantter by dilute acid. In certain otlier cases, a portion of the dermal 
®shibits these lines, whilst others are destitute of them. Their number 
differs widely in different species, from 8 to 210 or more; and it seems to 
ruy III the same species according to age; young Crayfish having from 6 to 
10, and large ones from 25 to 30 in corresponding portions of the shell. In 
bia speculations regarding the nature of these lines and the inti mate structure 
“^'“ •"liclhsubstance, M. Lavalle appears to me to have been altogether led 
^tniy by the insuffieimicy of his maguifving power, and probably also by 
j too great thickness of his sections, fie doseribea the substance of the 
emial layer a? made up of fdamciUs passing directly from one surface of the 
* cll to the other; these filaments being lioniogcneouu, and “ nregenting no 
are of a central canal." Nevertheless ho remarks, that on making a section 
to the surface, nothing is seen but “an immense number of little 
points ou a more transparent grotiiid.” This being precisely the ap- 
P^ranco of dentine, when examined under a low power, and cut iu a direc- 
*•>11 transverse to that of tin* tubuli, 1 cannot but feel surprised that M. 
i should have stopped short at this point. The application of a suffi- 
eiitly bigij po^ver to a verv thin vertical section of the shell brings into 
w^iDct new the existence of parallel tubuli (fig. 81) ; and if a section 
, - .. w. parallel tubuli (fig. 81), -. - 
surface be examined with a similar njagirifying power, the 
il» I ^’^huli are seen, surrounded by a clear space which indicates 
^of their walls, precisely as In ileutinc. ThU structure is par- 
in In the black extremities of the claws of the common Crab, 
of • k •‘'tertubular substance is quite pellucid in a thin section, and 
wiich the density is as great as that of many varieties of dentine. It 
, in the remainder of the shell; but from some difference in its 
do **^*^ 1 ^*^ constitution, the shell gcneraliv is of a much less dense and tena- 
wbil^h'^ ^racter, and the iud'ttubular substance has an opake chalky aspect, 
be It ® '■c*'y Ibin section of it impcrmeablo to light unless it 
labini'!^ Canada balsam, which then very commonly enters the 
184'^*^ them from being readily distinguishable. 
