278 EVENINGS AT THE MICROSCOPE. 



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 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 y 

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



* From the Greek Ixlvog (echinos) a hedgehog, and dspfia (derma) 

 sMn. A name given to these animals from their bodies being generally 

 armed with spines. 



