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: 2£o- 
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, 
fig. 183. -Pro- and some are parasitic. Such is Protamoeba, 
Fig. 183. 
tamceba pri¬ 
mitive. 
Class II.— Gregarinida. 
The Gregarinae, 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. 
