Class III.-SPOROZOA
Protozoa occurring as blood or intracellular parasites of other organisms. They possess no definite organs for locomotion or for ingestion of food. The typical reproduction takes place by the formation of spores and minute germs called sporozoites.
EXAMPLES: Gregarina, Coccidium, Plasmodium.
Class IV. INFUSORIA
Protozoa in which small filaments called cilia serve as organs of locomotion; these are distinguished from flagella by their much smaller size and in being present in great numbers, forming a fine covering over the whole of the body. The body protoplasm is always corticate.
EXAMPLE: Balantidium.
Class I. SARCODINA
This class, sometimes termed the Rhizopoda, includes parasitic amœbæ as well as free-living forms such as the Radiolaria and Foraminifera; these latter are of little interest to the student of tropical medicine. The distinctive characters of the amœbæ parasitic in man have already been given (p. 511). These are capable of independent amœboid locomotion by the alternate protrusion and retraction of protoplasmic processes called " pseudopodia."
Entamœba coli reproduces itself by binary fission in the ordinary way; this is called the vegetative or multiplicative phase. In addition to this a cystic or resting stage occurs, when conditions under which the parasite exists become adverse.
A sexual process, called autogamy, occurring within the cyst wall was described by Schaudinn, though recent authorities are inclined to doubt it. The process, as described, is as follows: The cyst contains at first but one nucleus, which divides into two. Each nucleus is then said to break up into chromidia and disappear, but to reappear later as a smaller and less distinct secondary nucleus on each side of the cell. Each secondary nucleus again divides into three; two of these again degenerate as reduction nuclei, while the third persists as a sexual or gameto-nucleus. The gameto-nuclei divide into four pronuclei, then two of each side conjugate and fuse, forming two nuclei or " synkarya" which subsequently divide into eight nuclei, in this manner forming eight amœbulæ within the cyst.
The end-product of the cystic stage of Entamœba histolytica possesses but four nuclei, and is known as the Entamœba tetragena
