With regard to creatures belonging to the lower degrees of organisation, we may first observe that worms, which live in the earth and remain in darkness and cold moisture, are imperfectly negatively coloured; worms bred in warm moisture and darkness are colourless; light seems expressly necessary to the definite exhibition of colour. 637. Creatures which live in water, which, although a very dense medium, suffers sufficient light to pass through it, appear more or less coloured. Zoophytes, which appear to animate the purest calcareous earth, are mostly white; yet we find corals deepened into the most beautiful yellow-red: in other cells of worms this colour increases nearly to bright red. 638. The shells of the crustaceous tribe are beautifully designed and coloured, yet it is to be remarked that neither land-snails nor the shells of crustacea of fresh water, are adorned with such bright colours as those of the sea. 639. In examining shells, particularly such as are spiral, we find that a series of animal organs, similar to each other, must have moved increasingly forward, and in turning on an axis produced the shell in a series of chambers, divisions, tubes, and prominences, according to a plan for ever growing larger. We remark, however, that a tinging juice must have accompanied the development of these organs, a juice which marked the surface of the shell, probably through the immediate co-operation of the sea-water, with coloured lines, points, spots, and shadings: this must have taken place at regular intervals, and thus left the indications of increasing growth lastingly on the exterior; meanwhile the interior is generally found white or only faintly coloured. 640. That such a juice is to be found in shell-fish is, besides, sufficiently proved by experience; for the creatures furnish it in its liquid and colouring state: the juice of the ink-fish is an example. But a much stronger is exhibited in the red juice found in many shell-fish, which was so famous in ancient times, and has been employed with advantage by the moderns. There is, it appears, in the entrails of many of the crustaceous tribe a certain vessel which is filled with a red juice; this contains a very strong and durable colouring substance, so much so that the entire creature may be crushed and boiled, and yet out of this broth a sufficiently strong tinging liquid may be extracted. But the little vessel filled with colour may be separated from the animal, by which means of course a concentrated juice is gained. 641. This juice has the property that when exposed to light and air it appears first yellowish, then greenish; it then passes to blue, then to a violet, gradually growing redder; and lastly, by the action of the sun, and especially if transferred to cambric, it assumes a pure bright red colour. 642.