A long gradation of cellular organization leads from the simplest primitive cells (monera) to the highest developed protists. While no morphological organization whatever is discoverable in the homogeneous plasma-body of the chromacea and bacteria, we find a composition from different parts in the highly differentiated body of the advanced protophyta (diatomes, siphonea) and protozoa (radiolaria, infusoria). The manifold parts of the unicellular organism, developed by division of work in the plasm, discharge various functions, and behave physiologically like the organs of the multicellular histona. But as the idea of "organ" in the latter is morphologically fixed as a multicellular part of the body, made up of numerous tissues, we cannot call these similarly functioning parts "organs of the cell," and had better describe them as organella (or organoids). The great majority of the protists are, in the developed condition, as actual individuals, equivalent morphologically to real nucleated cells. By means of adaptation to the most varied conditions and the inheritance of the properties thus acquired such a variety of unicellular forms has been evolved in the course of millions of years that we can distinguish thousands of living species, both of plasmodomous protophyta and plasmophagous protozoa. The number of known and named species is already as high as this in several distinct classes, as, for instance, in the diatomes of the primitive plants and the radiolaria of the primitive animals. These solitary living unicellulars, or "hermit-cells," may be called _monobia_. Many other protists have abandoned this original solitary life; they follow their social instincts and form communities or colonies of cells (_cœnobia_). These are usually formed by the daughter-cells which arise from the cleavage of a mother-cell remaining united after the division, and so on with the succeeding generations which come from their repeated segmentation. The following are the chief forms of these cœnobia: 1. GELATINOUS CŒNOBIA.--The social cells secrete a structureless mass of jelly, and remain associated in the common gelatinous mass, without actual contact. Sometimes they are regularly, at other times irregularly, distributed in it. We find cœnobia of this kind even among the monera, such as the _zooglœa_ of many bacteria and chromacea. They are common among the protophyta and protozoa. 2. SPHERICAL CŒNOBIA.--The cell-community forms a sort of ball, the cells lying close together at its surface, touching each other or even forming a continuous layer; such are _holosphæra_ and _volvox_ among the protophyta, _magosphæra_ and _synura_ among the protozoa. The latter are particularly interesting because they resemble the _blastula_, an important embryological stage of the metazoa, of which the simple, epithelial cell-layer at the surface of the hollow sphere is called the _blastoderm_ (or germinal membrane).