Now we shall proceed to show that the schemes of colouration follow out these structure-plans, and thus give additional force to the truth of the classification, as well as showing that, viewed on a broad scale, the present theory is a true one. We can, in fact, throw the whole scheme into a table, as follows:-- SYSTEMS OF COLOURATION. +--+-------------------------+------------------------+------------------+ | | System of Colouring. | Structure. | Sub-kingdoms. | +--+-------------------------+------------------------+------------------+ | |_A. No Axial Decoration._|_A. No Axial Structure._|_A. Invertebrata._| |1.| No definite system. | No definite organs. | Protozoa. | |2.| Radiate system. | Radiate structure. | Coelenterata, | | | | | Annuloida.| |3.| Segmental system. | Segmental structure. | Annulosa. | |4.| Marginate system. | Marginate growth. | Mollusca. | | | | | | | | _B. Axial Decoration._ | _B. Axial Structure._ | _B. Vertebrata._ | |5.| Axial system. | Axial structure. | Vertebrata. | +--+-------------------------+------------------------+------------------+ _Protozoa._ The protozoa are generally very minute, and always composed of structureless protoplasm. Their peculiarities are rather negative than positive, there being neither body segments, muscular, circulatory, nor nervous systems. Even the denser exterior portion (_ectosarc_) possessed by some of them seems to be rather a temporary coagulation of the protoplasm than a real differentiation of that material. Here, then, we have to deal with the simplest forms of life, and if colouration depends upon structure, these structureless transparent creatures should lack all colour-pattern, and such is really the case. Possessing no organs, they have no colouration, and are generally either colourless or a faint uniform brown colour, and through their colourless bodies the food particles show, often giving a fictitious appearance of colouring. To this general statement there is a curious and most telling exception. In a great many protozoa there exists a curious pulsating cell-like body, called the contractile vesicle, which seems to be a rudimentary organ, whose function is unknown. Here, then, if anywhere, traces of colouring should be found, and here it is accordingly found, for, though generally clear and colourless, it sometimes assumes a pale roseate hue. This may be deemed the first attempt at decoration in the animal kingdom, and it is directly applied to the only part which can be said to possess structure. Beautiful examples are plentiful in Leidy's magnificent volume on Freshwater Rhizopods.