SKELETONS OF OPHIURTDA AND CEINOIDEA. 



5P3 







o- 7 



Calcareous plate and claw of Astrophyton 

 (Euryale). 



The Spines with which the Arms of the species of Ophiocoma 

 (Brittle-star) are beset, are often remarkable for their beauty of 

 conformation ; those of 0. rosula, one of the most common kinds, 

 might serve (as Prof. E. Forbes justly remarked), in point of light- 

 ness and beauty, as 



models for the Spire of p IG 292 



a Cathedral. These are 

 seen to the greatest ad- 

 vantage when mounted 

 in Canada Balsam, and 

 viewed by the Binocular 

 Microscope with Black - 

 ground illumination. 



435. The Calcareous 

 Skeleton - is very highly 

 developed in the Crino- 

 idea ; 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 

 Caput Medusa;, a somewhat rare animal of the West Indian seas, 

 but also in a large proportion of the Fossil Crinoidea 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 sub- 

 stituted 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 struc- 

 ture is often most perfectly preserved.* 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 transverse section ; and the patterns, though formed 

 upon one general plan, are sufficiently diverse in different species 

 to enable these to be recognized by the examination of a trans- 

 verse section of a single joint of the Stem. 



436. 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 



* The Calcareous Skeleton even of living Echinodenns has a Crystalline 

 aggregation, as is very obvious in the more solid Spines of Bchinmnetrw, 

 &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 Areola? of the 

 Fossilized Skeleton has always the same Crystalline system with the 

 Skeleton itself, as is shown notmerely by the uniformity of their CJpa^age, 

 but by their similar action on Polarized Light. 



2 



