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 Amœba, 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 Amœba), 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:—