ON THE MICROSCOPIC STRUCTURE OP SHELLS. 
125 
at the base or attached end of the spine, since the portion winch forms the 
cup is composed, as in the Echiitm, of a colourless reticulated structure, 
whose areola are remarkably small, and this passes into the ordinary tissue 
of (he spine without any such break as might be expected to present itself 
if new growths were there interposed. This much, however, seems certain, 
that whatever additions these spines may receive in lengtli, they cannot be 
augmented in diameter, tliis being fixed in the first instance by the produc¬ 
tion ofthesohd calcareous cylinder, whldi forms the exterior of the spine. 
117. The same general plan of structure may be traced in the spiny ap¬ 
pendages of other Echinida ; such as the slender almost filamentary spines of 
the SpntangaccK, and the innumerable still more minute hair-like processes 
tttaclied to the shell of tJjo Clgpeasteria. Many of tliesc are extremely 
bcaatiful objects, when examined entire under the microscope without any 
preparation. 
118. It is interesting also to remark that the same calcareous reticulation 
presents itself in the curious PetUcellaria:, which is found upon tire surface 
of many Echinida, and has been so great a source of perplexity to /.oolo- 
gists. The complete conformity winch exists between the structure of their 
skeleton and that of the animal to which they are attached, ought in my 
opinion to remove any remaining doubt that they are truly appciuluges of it. 
In fwt, I can see no more reason for regarding tlieni as jiarasilcK, t!»an we 
consider the ambulacra! tubes. That they participate in the vitality of the 
body upon which they grow, is shown by this curious fact among others,— 
that when the skin of a living Eckhms la irritated with the point of a needle 
w pin, the small spines, and also the jmliccllaruB in the neighbourhood, all 
bead over towards the irritated spt»t, their movements being obviously di- 
to the removal of tlio source of irritation. This statement, first made 
by Sara, Las been denied by I’rof Forbes (British Starfishes, p. 159) ; but 
1 can bear personal testimony to the fact, having repeatedly triecl the expe- 
Rincnt upon a large and lively specimen oi Eckhms spkara, which I had 
under observation i'or several days •. 
The structure exhibited in the shell nf the Echinus is repeated In that 
genera which maybe regarded as the types of the principal 
^bdivisiona of this order; namely, Cklaris, Clypcastcr^ and Spatangus. 
here can Iherelbre be no reasonable doubt that it is universal throughout 
e group. Xo such differences present themselves, however, as would serve 
0 establish generic or even family distinctions; but the structure is so cha- 
^ttenstic of the class, that the minutest fragment of the skeleton of an 
iQoderm might be unhesitatingly rceoguked as such by the aid of the 
. structure U perfectly preserved, even after tlio shell 
« wen infiltrated with calcareous matter in the act of fussilization, and has 
oraeso completely mineralized, that the disposition to rhoiidmidul cleavage 
obtain a section In any other direction. The fossil 
onda of the oolite arc much better subjects for microscopical investi- 
^lotj than are those of the chalk; jslnce, beautiful os the latter are exter- 
conr^' j '"^^^ration by which they have been niitjcralized seems to have 
fled so much undissolved carbonate of lime, wiiich has been deposited 
amorphous state, that it is difficult to obtain transparent sections of 
c*sc»^»oiitrpe cvidenca} is of course to be preferred to negaUi>e. It vrill be 
aoW;S Agassiz feat! the temerity to dciiv Hut the Echmida conW use their 
for progression, because he had never seen them do so; a sttitcment very 
tte Pfof- Forbes {op. «/, p. i n). The large itjihtera, inonliotied ra 
near ♦ . “P ® by niemis of these tubes, and kept itself anchored 
tne top by the suckers at their catremity. 
