INFUSORIA. 19 



by cilia, or minute hairs, with which one end of the pear-shaped 

 gemmule is beset : these constantly keep up a rapid vibration, 

 and thus row the embryo sponge from place to place, until it 

 reaches a distant and suitable spot, where it quietly settles down, 

 and soon takes the form peculiar to its species. 



Were we to inform our young readers that flints have been 

 sponges, and that every flint wherewith, in many parts of the 

 country, the roads are paved, and which, before the invention of 

 lucifer matches, constituted almost the only means of obtaining 

 fire, had grown at the bottom of the sea, rooted upon rocks, and 

 sucking in the surrounding water through innumerable pores on 

 their surface, which conveyed through every part of their soft 

 texture materials for their subsistence we could scarcely expect 

 the assertion to be credited, at least without considerable hesita- 

 tion ; and yet no fact in natural history is more easily demon- 

 strated. Not only do the fragments of flints examined under 

 the microscope reveal the fossilized texture of the sponge, but 

 not unfrequently the shells of the animalcules upon which they 

 lived are found in their substance, and even portions of the sponge 

 itself, as yet unpetrified, are often contained in their interior. 



CHAPTER IV. 

 CILIATED ANIMALCULES. INFUSORIA.* 



T") ETURNING once more to our examination of the drop of 

 XV water which has already furnished us with lessons of such 

 interest, we find it still offering to our notice animalcules widely 

 different in their structure from any that we have as yet encoun- 

 tered. They are all of them, however, distinguishable by one 

 striking feature in their economy namely, that they are pro- 

 pelled through the water by means of vibratile cilia, which are 

 sometimes distributed over the entire surface of their bodies ; 

 while in others, these wonderful organs are restricted to certain 

 parts, more especially to the vicinity of the mouth. The posses, 

 sion of a moutJi, as the reader will at once perceive, is in itself an 

 important character, whereby they are broadly separated from 

 the mouthless Rhizopods. And when we add to this that they 



* Met with in stagnant water. 



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