DIVISIONS OF THE ANIMAL KINGDOM The primary division of the animal kingdom is into two groups, one including all those animals whose body is composed of a single cell, the other all those whose body is built up from many cells. (See CELL ; CYTOLOGY.) The first group, usually regarded as a phylum, the Protozoa (q.v.), may be defined as including all animals in which the body, when fully developed, consists of a single cell which carries out all the activities of a living animal; or of a colony of cells, exhibit ing either no differentiation, or, in certain cases, distinguished from one another by the setting apart of one or more cells for repro ductive purposes.
The group is very extensive, and the animals included within it vary extremely in size and in structure. The single cells which compose the entire bodies of most of its members may be far more elaborately organized than any of the cells of higher ani mals; they may contain one or many nuclei, and these nuclei may present a differentiation of function.
The cytoplasm may itself possess an elaborate structure, in cluding contractile myonemes, fibrillae and neuroid fibrils which control locomotary activities. It may possess a definite food track and permanent contractile vacuoles for excretion and the control of the water content of the organism. Indeed, it is far from improbable that some Protozoa are the degenerate descend ants of multicellular animals.
The life history of Protozoa is often very elaborate, and the group exhibits many stages in the development of a sexual mode of reproduction, from such simple cases as those in Ciliata, where two identical feeding individuals come into association and ex change nuclei, to a fully developed sexuality where a large egg is fertilized by a small mobile spermatozoa as in the malarial para site. It is, in fact, known that in some cases these gametes undergo a process of maturation of which the essential part is a halving of the number of chromosomes in the nucleus by a meiosis identical in principle with that which occurs in all multi cellular animals.
The multicellular animals, or Metazoa, are distinguished from Protozoa by the fact that their bodies are composed of a number of cells, which are not equipotent, but differ from one another in structure, function and origin. Each of these cells is associated with others of the same kind so as to form a tissue, and the tis sues fulfil definite functions in the life of the animal of which they form a part. A Metazoan animal is an individual, a unit whose cells are subordinated to the whole. Unlike the cells of Protozoa, they are not, in general, capable of a complete inde pendent existence, and cannot, unless they be germ cells, repro duce the whole body of which they form a part.
Thus the Metazoan body exhibits a differentiation, its separate functions being performed by definite cells or congeries of cells, whilst the diverse functions of a Protozoan are carried out by dif ferentiated portions of an undivided mass of cytoplasm.
It follows, as a necessary concomitant of the cellular differ entiation of a Metazoan body, that all such animals must undergo a definite course of embryonic development whereby, by repeated cell division and differentiation of the cells so formed, the adult body arises from the single cell, the zygote or fertilized egg cell in which the individual begins its existence. With very rare ex ceptions, all clearly secondary, all Metazoa reproduce sexually, the male and female gametes, the spermatozoa and ova, being formed in definite organs, the gonads, and possessing markedly divergent characters. The ovum is always relatively large, pos sesses some food reserves, and is the only functional cell of four formed by the last two divisions of the process of maturation. The spermatozoan is a small cell, in which the cytoplasm is found only in very small amount, and the food reserve is only large enough to enable it to swim for the short period which is neces sary to reach an ovum. It always possesses some locomotor apparatus and is usually of uniform and very characteristic structure.