THE SUB-KINGDOM CCELENTEBATA. 9 



curiously differ, in their greater mobility, want of 

 colour, and feebler tendency to undergo histolo- 

 gical change, from the more highly vitalised bod}^- 

 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 Gregariiice 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 ' 

 into horny matter, the fibrous skeleton of the 

 Sponges, the manducatory apparatus of Chilodon 

 and its allies, the carapace of the Arcellina and 

 Infusoria, and perhaps even their cilia, appear 

 to be produced; or, by 'deposition' of mineral 

 particles, withdrawn from the envii'onment, shells 

 and other hard structures have their origin. Nay 

 more, the diverse forms of Protozoa have the 

 power of appropriating certain elementary matters 

 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 

 the body of the same Sponge a corresponding 

 diversity of curiously wrought spicules may be 

 obtained. 



Thus, the naturalist, first struck by the varia- 



