LIFE HISTORIES OF THE PROTOZOA. 741 
III. THE RHIZOPODA. 
Structure and Habits. We have almost anticipated a definition 
of the Rhizopoda, of which the Ameeba, or Protean animalcule, is 
the simplest form, by our frequent references to the ‘* Amoeba- 
form” or “ Ameeba-like” stages in the Moners and Gregarinas. 
The Ameeba is the starting point, the unit of the nucleated Pro- 
tozoa, the primitive, ancestral form to which the members of the 
subkingdom may be reduced. Until the Monera were discovered 
the Ameeba was regarded as the lowest possible animal, 
With the form of the Monera, a shapeless mass of protoplasm, 
changing each instant, throwing out threads or larger protrusions 
of the body, the Ameeba possesses 
a distinct organ, the nucleus, and 
its body mass is divided into a 
clear cortical and a medullar 
granular mass; the outer highly 
contractile, the inner granular 
portion acting virtually as a stock 
of food. These granules, like the 
grains of chlorophyll in vegetable 
cells and Diatoms and Desmids, circulate in regular fixed cur- 
rents, according to J. H. Clark. (See Fig. 132, after Clark; the 
usual form of Ameba diffluens Ehrenberg, magnified 100 diam- 
eters ; the arrows indicate the course of the circulating food. The 
head end is knobbed, and within free from granules.) We have 
then in Ameeba :— 
1. A nucleus, probably representing the nucleus and ‘ovary of 
the Infusoria. 
2. A head and posterior end. 
3. A circulation analogous to that of the Infusoria. 
This animal, as we may justly call it, since it takes in living 
protozoans and rejects their shells, has the power of moving in 
a particular direction, one end of the body always advancing 
first; which indicates the rudiments of a nervous and muscular 
power; and can swallow, digest and circulate its food. Whether 
it gives out nitrogen and absorbs oxygen or not is unknown. It 
reproduces by self-division, and some allied forms by the produc- 
tion of monad-like, flagellate spores. 
The Ameeba is a fresh-water form, living on the stems and 
Fig. 132. 
4 
Ameeba. 
