892 ECHINODERMA 



models for the spire of a cathedral. These are seen to the greatest 

 advantage when mounted in Canada balsam, and viewed by the 

 binocular 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 between 

 the ordinary reticular structure of the shell and the peculiar sub- 

 stance of the tooth which in the adult tooth of the Echinus can 

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

 Ophiothrix may be mounted in balsam as a transparent object with 

 scarcely 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 sub- 

 sequent addition that the upper part of the tooth has a bone-like 

 character. 



The calcareous skeleton is very highly developed in the Crinoidea, 

 their stems and branches being made up of a calcareous network 

 closely resembling that of the shell of the Echinus. This is extremely 

 well seen, not only in the recent Pentacrinus asterius, a somewhat rare 

 animal of the 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 fossilisation by a mineral infiltra- 

 tion, which seems to have substituted itself for the original fabric 

 (a regularly crystalline cleavage being commonly found to exist in 

 the fossil stems of Encrinites, &c., 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 net- 

 work 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 transverse section. 2 



The minute structure of the shells, spines, and other solid parts 

 of the skeleton of Echinoderma can only be displayed by thin 

 sections made upon the general plan already described in Chapter VII. 

 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 to 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. An illus- 

 tration of a section cut from a spine of Echhwnietra is given in 

 fig. 673. 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 



1 The calcareous skeleton even of living Echiiioderms has a crystalline aggregation, 

 as is very obvious in the more solid spines of Echinoinetr(s, &c. ; 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 fossilised skeleton has always the same crystalline system 

 with the skeleton itself, as is shown not merely by the uniformity of their cleavage, 

 but by their similar action on polarised light. 



2 See figs. 74-76 of the Author's memoir on ' Shell Structure ' in the Report of 

 the British Association, 1847. 



