OM THE MICROSCOPIC STRUCTURE OJ? SHELLS, 
121 
111. The entire substance of the testa enclosing the bodies of the Echi- 
nid<E 13 made up of the structure now described ; and there is no difference 
between the internal and external portions of the shell, except in regard to 
the relatire size of the areol®. In fig. 62 we see a vertical section of the 
shell of it'/miMs, which exhibits the comparatively coarse texture of its inner 
fwrtion, a. Ou the other hand, the texture is peculiarlv close and firm in 
the tubercles which rise from the outer surface to support the spines. The 
structure of the calcareous framework which encloses the teeth Is exactly of 
the same nature with that of the shell. In the teeth, however, a different 
arrangcineiit is found, since the simple calcareous network svoiild be by no 
ra^ns adapt^ to the purposes to which those organs are applied. For a 
miuute description of their structure, with which my own observations cor¬ 
respond so far as they have extended, I must refer to Prof. Valentins ‘ Mo¬ 
nograph. It has been pointed out by that eminent observer, that the buc¬ 
cal membrane contains a delicate calcareous reticulation, strongly resembling 
that which 1 have delineated in fig. 66; and he has detected a aiost beautiful 
example of the same structure in the calcareous skeleton with which, as long 
since observed by Monro, the sucker at that extremity of each ambulacra! 
tube is furnished. Of his beautiful delineation of one of these discs, as it is 
^n when the animal matter of the tube has l)cen renilcred transparent by 
boding in a solution of potash, a portion showing half the disc U reproduced 
1 ^“’ The spines of the Echinm, Cidnris, See, present a structure essen¬ 
tially the same with that of the plates composing the testa; but they exhibit 
some peculiarities which are due to their mode of growth, and others w Inch 
seem intended to confer upon them additional strength. When we make a 
t ID transverse section of almost any spine belonging to the genus Echinusy 
we are at once struck with the existence of a series of concentric layers, 
aiTanged in a mauner that strongly reminds us of tiio layers of wood in the 
‘isogenous tree*. The nmiher of these layers is extreiuely va- 
Ifi: and, as 1 shall presonlly show, it depends not merely upon the age of 
6 spjne, but upon the part of its length from which the section is taken, 
aa spine iR filled up with the same kind of calcareous network 
1 which the plates of the shell are composed ; but it is usually of a 
^ character, the areolm being larger in proportion to the fibro-calca- 
structure which surrounds them, so that the network seems made up 
^ ^ of mere threads (fig. 66). 'This medullary centre is 
is d”li • ^ circular, of open spots, in which it 
Dose 1 ^ cursory examination, might be sup- 
Lf perfect transparency to be void spaces; but a closer exami- 
are7h * outer .surface of the spine, makes it evidoiit that they 
* circular row of (toHd ribs or plllar.s, which form the ex- 
c-xain’'* Their solidity becomes very obvious when we either 
^ ® of a spine whose substance is pervaded (as frequently hap- 
sninp K*^*^*?*^ colour, or when wc examine a very thin section of any 
.1 y P®*®nzed light. Around the first circle of these solid pillars we find 
ler layer of the fibro-caIcarc<ius network, which again is bounded by 
effecud by the yrogresjjvc consolidation of the fibrous element of the 
'l»m corn™posuShlc to trace out for some djiUimcc a fibre which has been 
lotToUuriiin^ deposit, and whieb retnaios soft at the other. (See 
* Qu.io'. Anatomy, p. cxlviii. el seq.) 
wntric y surpriscl that Prof. Vslcnlin alioulU have indicated the con- 
is “ •*« has done, both in his plates and Lis descripttons, since 
^ithont bekg'S*" d ^ transverse sections of manv spines when they are merely polished, 
