SEA-URCHINS AND SEA-CUCUMBERS 315 



forming a circle, whose plane is perpendicular to the direc- 

 tion 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- 

 colored 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 hollows, 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 with the minutest fragment of solid 

 substance, would, by testing it with his microscope, be 

 able at once to affirm with certainty whether it had be- 

 longed to an Echinoderm or not. And this uniformity 

 obtains in all the divers forms which the animals assume, 

 and in all the various organs which are strengthened by 

 calcareous deposits Crinoid, Brittle-star, Five-finger, Ur- 

 chin, Sea-gherkin, or Synapta; ray, plate, spine, sucker- 

 disk, lantern, pedicellaria, dumb-bell, wheel, or skin- 

 anchorwhenever we find calcareous matter, we invariably 

 find it honeycombed, and eroded, as it were, in this re- 

 markable fashion. 



