Unicellular animals, i. e. animals in which the organism is a single cell physiologically complete in itself.

An apparent exception to the above given definition is met with in the fusion-plasmodium of Mycetozoa, or the temporary fused state of some Heliozoa. The compound individual, however, comports itself in every way like a simple one, and there is no differentiation of function. The same statement is true of those colonial Protozoa in which individuals remain in organic connection. The sole example of physiological specialisation among Protozoa appears to be the Flagellate genus Vo/vox, where the power of reproduction is limited to a few individuals in each colony.

Notwithstanding their unicellular character, many Protozoa attain a high grade of complexity. The cell may be naked: it may protect itself by a superficial coagulated pellicle of protoplasm, or by differentiated cuticular structures. The latter may remain in close connection with the cell, or be completely detached from it. They may be gelatinous, formed of cellulose or a cellulose-like material, chitinoid, or composed mainly of calcium carbonate or silica. In other instances they consist of foreign bodies of very various character held together by a cement organic or inorganic. As a rule, the skeleton is a continuous whole, but in some Heliozoa and Radiolaria it is discontinuous, i. e. spicular. During the growth of the cell, the first-formed portions of the skeleton may become inclosed in its substance; a few Heliozoa and Radiolaria, however, possess a special organic internal skeleton.

The protoplasm of the body exhibits much variety: it may be of a similar character throughout, or a more or less permanent distinction may exist between the exterior and central parts. It is often described as having a reticulate structure, or more correctly a vesicular, i. e. with more fluid and less refractile particles imbedded in a denser and more refractile matrix. It is sometimes coloured, and the colour may be proper to it, or derived from the food. Pigment may be present, either coloured vacuoles, granules, or corpuscles. Among the last-named, special interest attaches to the chlorophyl bodies which occur in many freshwater forms: whether they are to be considered as proper to the organism, or instances of an association or symbiosis of a green Alga with an animal, similar to that of a yellow Alga with an animal, e.g. an Infusorian (note 3, p. 833), Radiolarian (p. 881). See the account of Symbiosis given pp. 242-4, the authorities quoted p. 245, and the notes on the occurrence of chlorophyl in the account of the different classes (pp. 833, 842, 843, 868, 901).

In the structure of the cell itself attention must be paid to the density of the protoplasm, to the mode of locomotion, of ingestion and digestion of food, to reserve and excretory products, and the nucleus.

Though the protoplasm is throughout the body of the organism of the same essential structure, yet its density varies, and the products of digestion, foreign bodies, etc., may when it is very fluid be distributed throughout it evenly; or when its density is greater they may be restricted to a central region (endoplasm), leaving a more or less pronounced clear border (exoplasm) in which contractility is very marked. Some Infusoria indeed possess special fibrils of contractile protoplasm (p. 834), and distinct muscular connections between individuals are met with in some Vorticellids (p. 834). Locomotion is effected by flowing or vibratile extensions of the protoplasm. As to the former, when the protoplasm is not confined by a rigid envelope and is very fluid or very dense, there may be an even flow of the protoplasm as a whole. But as a rule the flow is restricted to partial and changeable extensions or pseudopodia, which become more and more specialised in form, more and more stable as the protoplasm increases in density.

The pseudopodia may even lose their locomotor function and be vibratile (Biitschli, Z. W. Z. xxx. p. 271; Gruber, Z. W. Z. xxxvi. pp. 461, 462; Id. ibid. xli. p. 212; Biitschli, 1Protozoa,' pp. 123, 672-3, 440). A very large section of Protozoa move solely by vibratile processes, fine cilia or cilia-bundles, stouter and longer flagella, or vibratile membranes (membranellae s. pectinellae). Sometimes both modes of motion may be found in the same individual at different times, e.g. some Flagellata (p. 841; cf. p. 845) or in different stages of the life-history, e. g. Radiolaria.

As to the ingestion of food, it is procured either by a flow of protoplasm inclosing it, by means of pseudopodia, or by the action of vibratile processes, more rarely by special organs like the suctorial tentacles of A cinetaria. When it is carried out by vibratile processes, there is either a special spot, frequently a depression or tube, at which it is taken into the body, or a special organ to retain it like the collar of Choanoflagellata or the vacuolar process of a few Flagellata (p. 842). The pseudopodia in many instances exercise an instantaneously paralysing effect on motile organisms, e. g. Infusoria. Only a few Protozoa possess special weapons of offence such as the rods discharged from the oesophagus in some Infusoria, the trichocysts or miniature thread cells of others (p. 834), a Flagellate (p. 484), and Dinoflagellate (p. 851) of some Sporozoa (p. 864). But it is not certain that the trichocysts in all cases are of use in entrapping prey. The food-material consists either of living or dead animals or plants, and the Protozoon is then said to be holozoic; or it is organic food-material in solution, and the Protozoon is saprophytic; or finally, in some instances where chlorophyl is present, nutrition appears to take place as in plants, in other words the Protozoon is holophytic.

But the presence of chlorophyl need not necessarily lead to holophytic nutrition; how far it does so, is at present a moot point. The digestion of solid food is effected by contact with the protoplasm, or by a food-vacuole which may be formed either by the inclosure of some water with the food, or by the secretion of a liquid drop round it. There can be little doubt that whenever water is inclosed, the drop is modified by a secretion of the protoplasm; indeed it may be absorbed and a food-vacuole subsequently secreted. See Krukenberg, Vergleich. Physiol. Vortrage, i. p. 48; Greenwood, 'Digestion in Rhizopods,' Journal of Physiology, vii. 1886. The undigested residue is expelled at any spot when the cell is little differentiated, at a special spot or even by a special aperture when it is highly differentiated (some Infusoria). Excess of nutrition gives rise to reserve material, fat, albumen bodies, starch in some instances, especially in the presence of chlorophyl, glycogen (Barfurth, A. M. A. xxv. 1885, p. 314), or a starch-like substance known as paramylum (p. 843). Special excretory granules or crystals are sometimes found.