Note 91 — Chapter VII

Report on the Mollusca and Radiata of the Ægean Sea, and on their distribution, considered as bearing on Geology. By Edward Forbes, F.R.S., &c.—Reports of the British Association, vol. xii. Professor Forbes remarks:—“A comparison of the testacea, and other animals of the lowest zones, with those of the higher, exhibits a very great distinction in the hues of the species, those of the depths being, for the most part, white or colourless, while those of the higher regions, in a great number of instances, exhibit brilliant combinations of colour. The results of an enquiry into this subject are as follows:—

“The majority of shells of the lowest zone are white or transparent; if tinted rose is the hue, a very few exhibit markings of another colour. In the seventh region, white species are also very abundant, though by no means forming a proportion so great as the eighth. Brownish red, the prevalent hue of the brachiopoda, also gives a character of colour to the fauna of this zone; the crustacea found in it are red. In the sixth zone the colours become brighter, reds and yellows prevailing,—generally, however, uniformly colouring the shell. In the fifth region many species are banded or clouded with various combinations of colours, and the number of white species has greatly diminished. In the fourth, purple hues are frequent, and contrasts of colour common. In the second and third, green and blue tints are met with, sometimes very vivid; but the gayest combinations of colour are seen in the littoral zone, as well as the most brilliant whites.

“The animals of Testacea, and the Radiata of the higher zones, are much more brilliantly coloured than those of the lower, where they are usually white, whatever the hue of the shell may be. Thus the genus Trochus is an example of a group of forms mostly presenting the most brilliant hues both of shell and animal; but whilst the animals of such species as inhabit the littoral zone are gaily chequered with many vivid hues, those of the greater depth, though their shells are almost as brightly covered as the coverings of their allies nearer the surface, have their animals, for the most part, of a uniform yellow or reddish hue, or else entirely white. The chief cause of this increase of intensity of colour as we ascend is, doubtless, the increased amount of light above a certain depth.”—p. 172.