The microscope and its revelations . h sometimes parallel, usu-ally have more of a reticulated arrangement, and sometimesdilate into irregular lacunse or spaces excavated in the hardsubstance.* The Author is not prepared to speak with con-fidence on this point; but he is disposed to think that thestructure of the teeth is essentially the same as that of theshell, save in the interspaces of the network being muchnarrower; and that the appearance of tubuli (in which has not been able to make-out distinct walls) is duemerely to the elongation of these interspaces. 315. The calcare-ous
The microscope and its revelations . h sometimes parallel, usu-ally have more of a reticulated arrangement, and sometimesdilate into irregular lacunse or spaces excavated in the hardsubstance.* The Author is not prepared to speak with con-fidence on this point; but he is disposed to think that thestructure of the teeth is essentially the same as that of theshell, save in the interspaces of the network being muchnarrower; and that the appearance of tubuli (in which has not been able to make-out distinct walls) is duemerely to the elongation of these interspaces. 315. The calcare-ous plates which form ^^<^- less compact ske-letons of the Asfe-riada (star-fish andtheir allies) and ofthe Ophiurida (sand-stars and brittle-stars), have the sametexture as those ofthe shell of this presents it-self, too, in the spinesor prickles of theirsurface, when these (as in the large Goniaster equestris) arelarge enough to be furnished with a calcareous framework, * Lectures on Histology, vol. ii., p. Calcareous plate and elaw of Astrophyto(Euryale). 560 OF ECHINODEEMATA. and are not mere projections of the horny integument. Anexample of this kind, furnished by the Astropliyton (betterknown as the Euryccle), is represented in Eig. 239. Thespines with which the arms of the species of OpJiiocoma(brittle-star) are beset, are often remarkable for their beautyof conformation; that of 0. rosula, one of the most commonkinds, might serve (as Prof. E. Eorbes justly remarked) inpoint of lightness and beauty, as a model for the spire of acathedral. 316. The calcareous skeleton is very highly developed inthe Crinoidea; their stems and branches being made-up of acalcareous network, closely resembling that of the shell of theEchinus. This is extremely wxll seen, not only in the recentPentacrinus Caput Medusce, a somewhat rare animal of theWest Indian seas, but also in a large proportion of the fossilCrinoidea, whose remains are so abundant in many of theolder
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