278 EVENINGS AT THE MICROSCOPE. 



We now see a texture beautifully delicate ; they are formed 

 of calcareous substance as transparent as glass, and re- 

 flecting 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 light- 

 ness, 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 diverse forms which the animals assume, 

 and in all the various organs which are strengthened by 

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

 Urchin, Sea-gherkin, or Synapta; ray, plate, spine, 

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

 skin-anchor, — whenever we find calcareous matter, we 

 invariably find it honey-combed, and eroded, as it were, 

 in this remarkable fashion. 



Dr. Carpenter has described this texture so well, that I 

 shall not apologise for quoting his words to you, especially 

 as you will have an opportunity here of testing their cor- 

 rectness, by personal observation. " It is," he remarks, 

 " in the structure of that calcareous skeleton, which pro- 

 bably exists, under some form or other, in every member 

 of this class, that the microscopist finds most to interest 

 him. This attains its highest development in the Echinida, 

 in which it forms a box-like shell, or ' test,' composed of 



* From the Greek ex""« (eehinos) as hedgehog, and Sep^a (derma) shin. 

 A name given to these animals from their bodies being generally armed 

 with spines. 



