ON THE MICROSCOPIC STRUCTURE OP SHELLS. 
119 
cal form of this structure, from which the transition is easy towards either the 
more solid or the more open character which it elsewhere presents. When we 
obtain a very thin slice of one of these plates, taken parallel to its surface, we 
find that It is composed of a lamina apparently in itself destitute of structure, 
which is perforated with considerable regularity by apertures of a circular 
or oval form (fig. 61). The diameter of these apertures v.aries to a certain 
extait in different parts of the same shell; but from numerous measure- 
mente which I have made, I believe that it may !«■ stated as usually between 
I-450th and I-25(X)th of an iuch. These plates lie parallel to each other, but 
not in contact; for they are separated by little pillars, which rise up verti¬ 
cally from each plate to support the next, and which thus connect the dif¬ 
ferent plates whilst holding them apart from each other. The broken bases 
or ends of these minute pillars are very commonly to be seen ujxm the sur¬ 
faces of die perforated plates (fig. 63c}; and they are generally attached to 
points intensedlate betweou three of Uie apertures. The entire thickness of 
the shell is thus made up by the succession of a great number of these per¬ 
forated plates, connected by their intemiediatc rows of niiuiitc pillars. The 
successive plates are always so disposed, thnt (he oentrea of the perforations 
of one shall correspond with the intermediate solid structure of the next, as 
shown at otf, fig. 61, where small portions of another plate are seou beneath 
tlie principal one. This arrangement is obviously destined to impart in¬ 
creased strength to the structure, being analogous to that which wc meet 
with in the bee's comb, where the centre of tnt' bottom of the cell on one 
side is opposite the meeting-point of the walls of three cells on the other. 
Thus, in whatever direction we slice the shell of tl»o Echinus, we always 
with a sort of reticulated structure; for if our section is parallel to the 
surface of any one of the jdates, it brings into view one or more of the per¬ 
forated lauiinai just described, whilst, if it be perjieudicular to the surface, 
it passes vertically through the series of these lamiiue, which form a succes¬ 
sion of bands nearly parfillel to each other, and takes the direction of the 
pillars that connect tiunn with each other, these iiillars dividing the spaces 
between the latnintB into areolm of n tolerably regular form (fig. G2). The 
is thus of an extremely porous character, the quantity of solhl matter 
not being equivalent probably to more tliaii onc-third ol its bulk, and the 
having the freest communication with each other. Hence it happens 
that if a portion of a fractured edge, or any other part from which the in¬ 
viting membrane has been reinoveil, be kid upon fluid of almost any de¬ 
scription, the fluid will be immediately siickcd-up into the substance of the 
shell, and will add considerably to its weight. Thus a piece of the testa, 
which when dry weighed 10 grains, when thus saturated with water weighed 
13 grains. 
108. The substance of which the laminee and pillars arc composed is so 
Mlremely transparent, that when we have reduced .a section to such a degree 
of thinness as to contain a small number of layers of reticulation, it is easy, 
by a proper adjustment of the focus of the microscope, to bring cither one 
of them into distinct view ; the intervention of two or three, between the eye 
and the layer which is under examination, not having the effect of impeding 
the sight In fact, the opacity of a thicker layer, as ot the 
'■atire shell, is due, not so much to the inability of its jiroper substance to 
transmit light, as to the disjHjrsiou of tJie light by the numerous irregular re- 
Hcxiune and refractions which it will uodurco when passing tlirough several 
laininte of the reticular structure. And we find, accordingly, that when 
wturaled with a transparent Huid, such as water or (still better) Canada 
