. The microscope and its revelations. o dentine ; whilst the keel andenamel layer more resemble an oblique section of Pinna than anyother form of shell-structure. The calcareous plates which form the less compact skeletons ofthe Asteroidea (star-fish and their allies) and of the />]iii!i-i<I<;i(sand-stars and brittle stars) have the same texture as tlmse ofthe shell of Echinus. And thispresents itself, too, in the spines orprickles of their surface whenthese (as in the great (,oninsterpquestris or knotty cushion-star)are large enough to be furnishedwith a calcareous framework. Anexam]
. The microscope and its revelations. o dentine ; whilst the keel andenamel layer more resemble an oblique section of Pinna than anyother form of shell-structure. The calcareous plates which form the less compact skeletons ofthe Asteroidea (star-fish and their allies) and of the />]iii!i-i<I<;i(sand-stars and brittle stars) have the same texture as tlmse ofthe shell of Echinus. And thispresents itself, too, in the spines orprickles of their surface whenthese (as in the great (,oninsterpquestris or knotty cushion-star)are large enough to be furnishedwith a calcareous framework. Anexam]tie of this kind, furnished bythe Astrophyton, is represented infig. 679. The spines with which Pm 679 Calcareous 1)lat,, aml elawthe arms of the species ofOpMothrix of AstnmJii/tdii. ( brittle star ) are beset are often remarkable for their beauty of conformation ; those of . penta-pJvyllwm, one of the most common kinds, might serve (as ProfessorE. Forbes justly remarked), in point of lightness and beauty, as. 892 ECHINODERMA models for the spire of a cathedral. These are seen to the greatestadvantage when mounted in Canada balsam, and viewed by thebinocular microscope with black-ground illumination. It is inter-esting to remark that the minute tooth of Ophiothrix clearly exhibits,with scarcely any preparation, that gradational transition betweenthe ordinary reticular structure of the shell and the peculiar sub-stance of the tooth which in the adult tooth of the Echinus canonly be traced by making sections of it near its base. The tooth ofOphiothrix may be mounted in balsam as a transparent object withscarcely any grinding down ; and it is then seen that the basal por-tion of the tooth is formed upon the open reticular plan characteristic-of the shell, whilst this is so modified in the older portion by subsequent addition that the upper part of the tooth has a bone-likecharacter. The calcareous skeleton is very highly developed in the Crinoidea,their stems and branches being m
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
