THE SUB-KINGDOM CCELENTERATA. 9 



ire 



curiously differ, in their greater mobility, want of 

 colour, and feebler tendency to undergo histolo- 

 gical change, from the more highly vitalised body- 

 mass within, it were not difficult to effect a natural 

 transition. In the Sponges structural relations 

 akin to those just mentioned are still more easily 

 to be traced. In the Gregarince an external en- 

 velope becomes sufficiently distinct from the 

 granular or vacuolated protoplasm which it 

 bounds. And in the Infusoria the more contrac- 

 tile body-substance not merely serves to enclose 

 a softer sarcode, but is itself protected by a cuticu- 

 lar covering, on which the styles and cilia are borne. 

 ^' But the changes which the sarcode substance 



^ undergoes are not simply structural or mechanical. 

 * Other modifications, of a more purely chemical 



nature, may either accompany or replace the pro- 

 ^ cesses above mentioned. Thus, by € conversion ' 

 l ^l into horny matter, the fibrous skeleton of the 

 ^ Sponges, the manducatory apparatus of Chilodon 



and its allies, the carapace of the Arcellina and 

 iter Infusoria, and perhaps even their cilia, appear 

 1 it to be produced ; or, by ' deposition ? of mineral 

 the particles, withdrawn from the environment, shells 



na! and other hard structures have their origin. Nay 



ilni more, the diverse forms of Protozoa have the 



power of appropriating certain elementary matters 

 i|r to the exclusion of others. The Polycystine 



sculptures its own siliceous shell ; the Foramini- 

 fer, living beside it, a calcareous one, not less 

 ^ complex or beautiful ; while from various parts of 



f the body of the same Sponge a corresponding 



diversity of curiously wrought spicules may be 

 - obtained. 



| e ' Thus, the naturalist, first struck by the varia- 



