160 PROTOZOA. 



CLASS INFUSORIA. 



These aiiinialcnles were first observed by Lewenhock in 1675 ;. 

 and our present knowledge is cliiefly due to the labors of Ehren- 

 berg, Pritchard, Dujardin and Bailey. The later researches have 

 tended to diminish the numbers of the so-called Infusoria ; many 

 forms have been shown to be only larval worms, while whole 

 genera (as DiatomacicB and BaciUarioB) have assumed under the 

 microscope a vegetable character. They are distinguished by 

 having bodies of a definite form, by moving chiefly by means of 

 vibratile organs (cilia), instead of pi'olongations and digitations 

 of the sarcode, and by the possession of a mouth and gullet and 

 rudiments of digestive, circulatory and reproductiv^e organs. 



Infusoria inhabit both fresh and salt waters, and are not found 

 fossil, — the fossil organism often called Infusorial, being Fora- 

 minifers, Polycystines and Diatoms. 



CLASS GREGARINIDA. 



These are parasitic forms without mouth or power of emitting 

 pseudopodia. They have a definite cell-wall and nucleus, but no 

 contractile vacuole. They reproduce by encysting and produc- 

 tion of moner-like young which undergo metamorphosis. 



The Gregarinida resemble worms, and are internal parasites of 

 various animals, particularly the earth-worm and cockroach. 

 They are sometimes ranked as the lowest of the subkingdom, 

 but they may be forms degraded by theii- mode of life. Having 

 no hard parts they do not occur fossil. 



