Fig. 52.—The Amœba, feeding
The amœba is an omnivorous feeder, but subsists mainly on vegetable organisms, especially on diatoms and other minute algæ; and the siliceous skeletons of the former may often be seen within the body of the animal, under the high power of a microscope.
The multiplication of the amœba is brought about by a process of fission or division. At first the nucleus divides into two, and then the softer protoplasm contracts in the middle, and finally divides into two portions, each of which contains one of the nuclei. The two distinct animals thus produced both grow until they reach the dimensions of their common progenitor.
Fig. 53.—The Amœba, dividing
All the protozoons resemble the amœba in general structure and function; but while some are even simpler in organisation, others are more highly specialised. Some, like the amœba, are unicellular animals; that is, they consist of a single, simple speck of protoplasm; but others live in colonies, each newly formed cell remaining attached to its parent cell, until at last a comparatively large compound protozoon is formed.
The sub-kingdom is divided into several classes, the principal of which, together with their leading characteristics, are shown in the following table:—
1. Rhizopods:—Body uniform in consistence. Pseudopods protruded from any point.
2. Protoplasta:—Outer protoplasm slightly firmer in consistence. Pseudopods protruded from any point. (Often grouped with the Rhizopods.)