SEA-URCniNS AND SEA-CUCUMBERS. 319 



are presently hurled away in the same plane ; forming 

 a circle, whose plane is perpendicular to the direction 

 of the spine. The surface upon which these cilia are 

 set is a transparent gelatinous skin, of extreme tenuity, 

 stretched tightly over the solid portion, which it com- 

 pletely covers, and studded with minute oval orange- 

 coloured grains. 



The substance of which the spines are composed is 

 best seen by crushing a few of these organs into frag- 

 ments. We now see a texture beautifully delicate ; 

 they are formed of calcareous substance as transparent 

 as glass, and reflecting the light like that material ; 

 hard but very brittle ; clear and solid, with a fibrous 

 appearance in some parts, but in others excavated 

 into innumerable smooth rounded cavities which join 

 each other in all possible ways. It is to this structure 

 that the spine owes its strength, its lightness, and its 

 brittleness. 



This arrangement of the calcareous deposit in a sort 

 of glass full of minute inter-communicating liollows, is 

 very peculiar, but it is invariably found in the solid 

 parts of this class of animals ; so that the experienced 

 naturalist, on being presented wuth the minutest frag- 

 ment of solid substance, would, by testing it with his 

 microscope, be able at once to afBrm with certaint}-, 

 whether it had belonged to an Ecliinoderm or not. 

 And this uniformity obtains in all the divors forms 

 which the animals assume, and in all the various 

 organs mIucIi are strengthened by calcareous deposits 

 — Crinoid, Brittle-star, Five finger, Urchin, Sea-gher- 

 kin, or Synapta ; — ray, plate, spine, sucker-disk, lan- 

 tern, pedicellaria, dumb-bell, wheel, or skin-anchor — 

 whenever we find calcareous matter, we invariably 



