Turning now to the corals, which are simply like colonies of single anemonies with a stony skeleton, we have quite a different arrangement of hues. No sight is more fascinating than that of a living-coral reef, as seen through the clear waters of a lagoon. The tropical gardens ashore cannot excel these sea-gardens in brilliancy or variety of colour. Reds, yellows, purples, browns of every shade, almost bewilder the eye with their profusion; and here again we find structural decoration carried out to perfection. The growing points of white branching corals (Madrepores) are frequently tipped with vivid purple, and the tiny polyps themselves are glowing gem-stars. In the white brain-corals, the polyps are vivid red, green, yellow, purple and so on; but in almost every case vividly contrasting with the surrounding parts, the colour changing as the function changes.
The Alcyonariæ, which include the sea-fans, sea-pens, and the red coral of commerce, practically bring us to the end of the Cœlenterata, and afford us fresh proof of the dependence of colour upon structure and function. The well-known organ-pipe coral (Tubipora musica) is of a deep crimson colour, and the polyps themselves are of the most vivid emerald green, a contrast that cannot be excelled. Almost equally beautiful is the commercial coral (Corallium rubrum) whose vivid red has given a name to a certain tint. In this coral the polyps are of a milk-white colour.
It must be remembered that in these cases the colour seems actually to be intentional, so as to form a real and not merely an accidental contrast between the stony polypidom and the polyp, for the connecting tissue (cœnosarc) is itself as colourless as it is structureless.
Gathering together the facts detailed in this chapter we find:—
1. That the Protozoa are practically colourless and structureless.
2. That in those species which possess a rudimentary organ (contractile vesicle) a slight decoration is applied to that organ.
3. That in the Cœlenterata 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.
[CHAPTER VIII.]
Details of Protozoa.
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