240 COMPARATIVE ZOOLOGY. 



the essential signs of life. The usual methods of repro- 

 duction are self-division and budding. 



The subkingdom may be divided into four classes : Mo- 

 nera, Gregarinida, Rhizopoda, and Infusoria. 



CLASS I. Monera. 



These simplest living beings are organless 

 bits of protoplasma, with no distinction of 

 layers, round when at rest, and with pseu- 

 dopodia when active. They are all aquatic, 

 Fio.i83.-Pro- and some are parasitic. Such is Protamoeba, 



tamcebapri- -.-,. -. Q o 

 mitiva. -big. 18d. 



CLASS II. Gregarinida. 



The Gregarinse, discovered by Dufour in 1828, are 

 among the simplest animal forms of which we have any 

 knowledge. They closely resemble a cell, or microscopic 

 egg ; the only organ is a nucleus, suspended in extremely 

 mobile granular matter; and the most conspicuous signs 



Fro. ISl.Gregarina gigantea, highly magnified : a, nucleus. 



of life are the contraction and lengthening of the worm- 

 like body. They feed by absorption, and are all parasites, 

 living in the alimentary canal of higher animals ; as in 

 the Cockroach, Earth-worm, and Lobster. The name is 

 derived from the fact that they occur in large numbers 

 crowded together. 



CLASS III. Rhizopoda. 



The Ehizopods are characterized by the power of throw- 

 ing out at will delicate processes of their bodies, called 

 pseudopodia, or false feet, for prehension or locomotion. 



