146 THE MICROSCOPE AND ITS REVELATIONS. 



clearly exhibits, with scarcely any preparation, that gradational trans- 

 ition between the ordinary reticular structure of the shell and the peculiar 

 substance of the tooth, which, in the adult tooth of the Echinus, can 

 only be traced bv making sections of it near its base. The tooth of 

 Ophiocoma may be mounted in balsam as a transparent object, with 

 scarcely any grinding down; and it is then seen that the basal portion of 



the tooth is formed upon the open re- 

 no. 372. ticular 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-like 

 character. 



537. The calcareous skeleton is very 

 highly developed in the Crinoidea; 

 their stems and branches being made-up 

 of a calcareous network closely resem- 

 bling that of the shell of the Echinus. 

 This is extremely well seen, not only in 



Calcareous plate and claw of Astrophyton the 1'CCent PentdCrinUS Caput MeduSCB, 



(Euryaie). ft somew h a t ra re animal of West Indian 



seas, but also in a large proportion of 



the fossil Crinoids, whose remains are so abundant in many of the older 

 Geological formations; for notwithstanding that these bodies have been 

 penetrated in the act of fossilization by a Mineral infiltration, which 

 seems to have substituted itself for the original fabric (a regularly-crys- 

 talline cleavage being commonly found to exist in the fossil stems of 

 Encrinites, etc., as in the fossil spines of Echinida), yet their organic 

 structure is often most perfectly preserved. 1 In the circular stems of 

 Encrinites, the texture of the calcareous network is uniform, or nearly 

 so, throughout; but in the pentangular Pentacrini, a certain figure or 

 pattern is formed by variations of texture in different parts of the trans- 

 verse section. 2 



538. The minute structure of the Shells, Spines, and other solid 

 parts of the skeleton of Echinodermata can only be displayed by thin 

 sections made upon the general plan already described ( 192-195). 

 But their peculiar texture requires that certain precautions should be 

 taken; in the first place, in order to prevent the section from breaking 

 whilst being reduced so the desirable thickness; and in the second, to 

 prevent the interspaces of the network from being clogged by the particles 

 abraded in the reducing process. A section of the Shell, Spine, or other 

 portion of the skeleton should first be cut with a fine saw, and be rubbed 

 on a flat file until it is about as thin as ordinary card, after which it 

 should be smoothed on one side by friction with water on a Water-of-Ayr 

 stone. It should then, after careful washing, be dried, first on white 

 blotting-paper, afterwards by exposure for some time to a gentle heat, so 



1 The calcareous skeleton even of living Echinoderms has a crystalline aggre- 

 gation, as is very obvious in the more solid spines of Echinometrce, etc. ; for it is 

 difficult, in sawing these across, to avoid their tendency to cleavage in the oblique 

 plane of calcite. And the Author is informed by Mr. Sorby, that the calcareous 

 deposit which fills up the areolae of the fossilized skeleton has always the same 

 crystalline system with the skeleton itself, as is shown not merely by the uni- 

 formity of their cleavage, but by their similar action on Polarized light. 



2 See Figs. 74-76 of the Author's Memoir on " Shell Structure" in the Report 

 of the British Association for 1847. 



