2. That in those species which possess a rudimentary organ (contractile vesicle) a slight decoration is applied to that organ. 3. That in the Coelenterata the colouration is directly dependent upon the structure. 4. That in transparent animals the colouration is applied directly to the organ whether it be internal as in the canals or ovaries, or external, as in the eye-specks. 5. That in opaque animals, as in the sea-anemonies, the colouring is entirely external. 6. That it is very variable in hue, but not in pattern. 7. That the most highly differentiated parts (tentacles, eye-specks), are the most strongly coloured. 8. That in the corals an emphatic difference occurs between the colour of the polypidom (or "coral") and the polyp. [Illustration] CHAPTER VIII. DETAILS OF PROTOZOA. The Protozoa are divided into three orders. I.--_Gregarinidæ._ II.--_Rhizopoda._ III.--_Infusoria._ I. The _Gregarinidæ_ consist of minute protozoa, parasitic in the interior of insects, &c., and like other internal parasites are colourless, as we should expect. II. The _Rhizopoda_ may, for our purpose, be divided into the naked forms like _Amoeba_, and those which possess a skeleton, such as the Radiolaria, the Foraminifera and the Spongia. Of these the naked forms are colourless, or uniformly tinted, excepting the flush already described as emphasizing the contractile vesicle. The _Foraminifera_ are the earliest animals that possess a skeleton or shell, and though generally very small, this shell is often complex, and of extreme beauty, though their bodies retain the general simplicity of the protozoa, indeed, they are said to possess no contractile vesicle. Still the complexity of their shells places them on a higher level than the naked rhizopoda. In these animals we find the first definite colour, not as a pattern, but as simple tinting of the protoplasm. The general hue is yellowish-brown (as in _Amoeba_), but deep red is not uncommon. The deepest colour is found in the oldest central chambers, becoming fainter towards the periphery, where it is often almost unrecognisable.[19] The _Radiolaria_ are minute organisms with still more complex skeletons, and are considered by Haeckel[20] to be more highly organized than the preceding order. They consist of a central portion containing masses of minute cells, and an external portion containing yellow cells. Here we have the first differentiation of parts in the external coating and internal capsule, and side by side with this differentiation we find colour more pronounced, and even taking regional tints in certain forms. We may notice the following genera as exhibiting fine colour:-- _Red._ Eucecryphalus, Arachnocorys, Eucrytidium, Dictyoceras. _Yellow._ Carpocanium, Dictyophimus, Amphilonche.