Protozoa from the Greek words (Gr. πρώτος, first, and ζώον, living thing).
ANIMALS are composed of living material, which is called protoplasm. Except in the simplest animals the protoplasm is arranged in the form of separate units or cells, each a minute piece of living matter. A complex animal is composed of a vast number of these cells, which are of different kinds and are combined to form the fabric pf the body, much in the same way as bricks, beams, and tiles are combined to form a house. It is the custom to regard the body of the simplest animals as though it were one cell ; that is to say, it may be compared, though not exactly, with one of the units which combine to form a complex animal. The simplest animals are, therefore, described as unicellular, while others are called multicellular. Unicellular animals are, as a group, named the Protozoa. Multicellular animals are named Metazoa.
Weismann, in his classical essays on the germ-plasm, argued in favor of the view that the protozoa were potential germ cells, and, since new individuals arise by division of the parent cell into two or more parts, that natural death does not occur.
The protozoa are consequently also potentially immortal. The metazoa, on the other hand, possessed a large amount of somatic substance which always dies a natural death.
It had often been pointed out that a PROTOZOON, although consisting of but a single cell, performs most of the physiological activities characteristic of the larger, complex Metazoa, and that certain parts of the PROTOZOON are recognizably concerned with the performance of certain definite functions. The fundamental difference, then, between the one-celled and the many-celled animals is that the differentiated structures in the former are not separated from one another by cell walls as in multicellular organisms.
Whether all protozoa possess a body which can be considered as specialized and set aside for reproduction purposes, as the germ plasm theory requires, is a question upon which authorities differ.
The life history of the fresh water rhizopod, Arcella vulgaris, will serve to illustrate this (Hertwig, 1899 ; Eipatiewsky, 1907; Swarczewsky, 1908; Calkins, 1911).
The single nucleus of the young Arcella divides to form two primary nuclei (N) ; chromatin from these migrates out and forms a layer near the periphery (Ch) the " chromidial net " of Hertwig. This chromatin substance in the mature individual produces hundreds of secondary nuclei (n) , each of which is cut off, with a small amount of the surrounding cytoplasm, from the others, thus becoming a swarm spore. The swarm spores escape from the mouth of the parent cell ; whereas the two primary nuclei and a portion of the cytoplasm not used up in the forma tion of the swarmers die. The swarmers are not all alike, being of two sizes ; the larger, which may be called macrogametes, and which correspond to the eggs of the METAZOA, fuse with the smaller micro gametes. The zygotes which result develop into normal Arcellce. The swarmers may be supposed to represent the germinal protoplasm, of which, as in metazoan germ cells, the chromatin content may be considered the essential portion. The conditions during reproduction in other PROTOZOA may also be explained in this way, so that germinal and somatic protoplasm can be distinguished as in the metazoa.
The discovery of the chromidia in PROTOZOA led to the formulation of the hypothesis of binuclearity. Believers in this hypothesis maintain that each cell contains both a somatic and propagatory nuclear material which, as a rule, are united into one amphinucleus. The somatic nuclear material controls vegetative functions ; the propagative portion serves only for the propagation of new individuals. Separation occurs rarely except in certain PROTOZOA, where, as in Paramecium, the propagative substance is represented by the micronu cleus, the somatic by the macronucleus. Since the chromatin is the essential substance concerned in the binuclearity hypothesis, the term dichromaticity has been suggested as more appropriate, and the two kinds of chromatin involved have been called idiochromatin, which is reproductive in function, and trophochromatin, which is vegetative in function.
The hypothesis had not gained many adherents and was considered of doubtful value by eminent protozoologists (Dobell, 1908).