If the principle of the dependence of colour-pattern upon structure, enunciated in the preceding pages be sound, we ought to find certain great schemes of colouration corresponding to the great structural subdivisions of the animal kingdom. This is just what we do find; and before tracing the details, it will be as well to group the great colour-schemes together, so that a general view of the question can be obtained at a glance. The animal kingdom falls naturally into two divisions, but the dividing line can be drawn in two ways. If we take the most simple classification, we have:-- 1. _Protozoa_, animals with no special organs. 2. _Organozoa_, animals possessing organs. Practically this classification is not used, but we shall see that from our point of view it is a useful one. In the most general scheme the divisions are:-- 1. _Invertebrata_, animals without backbones. 2. _Vertebrata_, animals with backbones. The invertebrata are divided into sub-kingdoms, of which the protozoa form one. These protozoa possess, as it were, only negative properties. In their simplest form they are mere masses of protoplasm, even lacking an investing membrane or coat, and never, even in the highest forms, possessing distinct organs. It is this simplicity which at once separates them entirely from all other animals. The other sub-kingdoms are:-- _Coelenterata_, of which the jelly-fishes are a type; animals possessing an alimentary canal, fully communicating with the general cavity of the body, but without distinct circulatory or nervous systems. _Annuloida_, of which the star-fishes are a type; animals having the alimentary canal shut off from the body-cavity, and possessing a nervous system, and in some a true circulatory system. _Annulosa_, of which worms, lobsters, and insects are types; animals composed of definite segments, arranged serially, always possessing true circulatory and nervous systems. _Mollusca_, of which oysters and whelks are types; animals which are soft-bodied, often bearing a shell, always possessing a distinct nervous system and mostly with a distinct heart. In old systems of classification, the _Coelenterata_ and _Annuloida_ were united into one sub-kingdom, the _Radiata_, in consequence of their radiate or star-like structures. As colouration, according to the views here set forth, depends upon structure, we may classify the Invertebrata thus:-- Protozoa Structureless. Coelenterata } Radiata. Radiate structure. Annuloida } Annulosa Segmented " Mollusca Marginate " The mollusca are said to be marginate in structure because, in those possessing shells--the mollusca proper--the shell is formed by successive additions to the margin or edge of the shell, by means of the margin of the mantle, or shell-secreting organ.