124 
REPORT—1847. 
mantle of the Mollusca, furnish an extraordinary supply of the requisite m- 
terials, when they are required for the reparation of an injury*. 
116. In the spines of Cidaris a different arrangement seems topre?^:^ 
which difference I believe that Professor Gmiit was the first to drawattpoti-s 
(Outlines of Comparative Anatomy, p. 21). “ In some of tlieae exl-jud 
spines of Echinida” he says, “the growth appears to be effected by lb 
addition of calcareous matter only to the jiroxininl extremity, or fixed «i> 
of the spines. The lines of growth do not then cooveige aod men i 
longitudinal sections, but diverge and tcrininale at the sides nf Uie?|tina 
This structure is seen in the spines of Cidarit pistidaris. TLe cunipKi 
portion of the spine formed within the capsule is seen occupying the mid* 
of its whole extent; and the 8ucccssi%’G layers of growth are obscfW 
extending from this central portion to the sides along the whole apW 
This description, however, having been given at a time when nothing ** 
known of the minute strurture of the shell or its appendages, is farfaa 
being accurate, as will appear from the following account of the apjvr- 
ances presented when thin sections of tlie spines of Cidaris areexwm»I 
with tlie microscope. Their principal sul>staiicc is made up of the sm 
calcareous areolar structure as that of which the shell is composed; I# 
this presents none of the concentric arrangement so beautifully tlisplaj'’* 
by transverse sections of the spintw of Evhini. The central portion uaiufr 
has a larger or looser reticulation than the external; but there is do ilefifliie 
boundary betw’cen the two, the large areolao of the central portion beconiin,' 
gradually contracted, until they pass into the average of those which luah 
xip the principal part of the area of the section. As there is oothingiili** 
concentric arrangement of successive layers, so there are but very slight ioil- 
cations of the longitudinal laniellm, v'hose radiating arrangemrflt i> so fh*’ 
racteristic in the spines of Echini, A transverse section shows that aN 
certain Inu's passing from the centre to tlie circumference, the calcafK"-' 
network seems to have a greater tendency to solidity than in the interremn? 
spaces, presenting, in fact, very much the appearance of those portions whivi 
are seen iii fig. C9 to connect the 8<ilid ribs of one layer with those of another 
but m no part of the longitudinal section do we see this network pas*®? 
into a solid p ate or rib \ nor have I been able to detect, either in tnnsTs^ 
or in longitudmal sections, the least trace of the interruption of the vfdi. 
^ucture by lines of growth. But the areolar structure of the spinf f 
Orfam IS completely encased by a cylindrical covering of a solid apparr"'^' 
homogeneous substance, chiefly calcareous; and it is tliis which is veil 
munly raised up in ndges on its exterior. This substance is traveised]- 
lines, which puss from the interior substance to the external surf8ce ®f‘^ 
S thP n however not being exactiv at right angles to the iw 
of the spine, but being commonly a little oblique. These I presume M J* 
‘"dicate tlie successive l.ye« 
Lin of the spioe; but when they 
magnifying power in thin sections, transverse or 
tudmal, of the spine, they are at unco perceived to be tvbet of irivgui*^* 
iri^LhTh’ c«»s»>erablr size, wbieh pass from the esteroal 
w t structure. I am o«able 
mriiL fnTi the spines of Cnfamare 
thin?iH.!“' suffif-ient indications of .successive formation; h-* 
think It improbable that the additions can be made (as Dr. Giant 
nity LtpeSmeSn^'! having 
tioh can be Sven see what other probable ex? 
“>* oe given of the appearances above described. 
