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 w T hen active. They are all aquatic, 

 fig. 183— Pro- and some are parasitic. Such is Protamoeba, 



tamoeba pri- j^. i QO 

 mitiva. -T ] g- -loo. 



Class II. — Gregarinida. 

 The Gregarinj£, 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 



Fig. 184:. — 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 Rhizopods 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. 



