Many species of the simplest chromacea live as monobia (individually). When the tiny plasma globules have split into two equal halves by simple segmentation, they separate, and live their lives apart. This is the case with the common, ubiquitous chroococcus. However, most species live in common, the plasma granules forming more or less thick cœnobia, or communities or colonies of cells. In the simplest case (_aphanocapsa_) the social cytodes secrete a structureless gelatinous mass, in which numbers of blue-green plasma globules are irregularly distributed. In the _glœocapsa_, which forms a thin blue-green gelatinous deposit on damp walls and rocks, the constituent cytodes cover themselves immediately after cleavage with a fresh gelatinous envelope, and these run together into large masses. But the majority of the chromacea form firm, threadlike cell communities or chains of plastids (catenal cœnobia.) As the transverse cleavage of the rapidly multiplying cytodes always follows the same direction, and the new daughter-cytodes remain joined at the cleavage surfaces, and are flattened into discoid shape, we get stringlike formations or articulated threads of considerable length, as in the oscillaria and nostochina. When a number of these threads are joined together in gelatinous masses, we often get large, irregular, jelly-like bodies, as in the common "shooting-star jellies" (_nostoc communis_). They attain the size of a plum. In view of the extreme importance which I attach to the chromacea as the earliest and simplest of all organisms, it is necessary to put clearly the following facts with regard to their anatomic structure and physiological activity: 1. The organism of the simplest chromacea is _not_ composed of different organella or organs; and it shows no trace of purposive construction or definite architecture. 2. The homogeneous tinted plasma granule which makes up the entire organism in the simplest case (_chroococcus_) exhibits no plasma structure (honeycomb, threads, etc.) whatever. 3. The original globular form of the plasma particle is the simplest of all fundamental types, and is also that assumed by the inorganic body (such as a drop of rain) in a condition of stable equilibrium. 4. The formation of a thin membrane at the surface of the structureless plasma granule may be explained as a purely physical process--that of surface strain. 5. The gelatinous envelope which is secreted by many of the chromacea is also formed by a simple physical (or chemical) process. 6. The sole essential vital function that is common to all the chromacea is self-maintenance, and growth by means of their vegetal metabolism, or plasmodomism (=carbon assimilation); this purely chemical process is on a level with the catalysis of inorganic compounds (chapter x.). 7. The growth of the cytodes, in virtue of their continuous plasmodomism, is on a level with the physical process of crystal growth.