.1. The nature of the colour changes shown by Sepia officinalis L. in different environments is discussed.

2. Several patterns in the coloration of the animal, each appearing under certain specific conditions of stimulation, are described.

3. Each pattern is formed by a pattern of expansion and contraction of the chromatophores, supplemented by reflection from the immobile iridophores which lie below them.

4. The state of expansion of each chromatophore is determined by its musculature, which is under nervous control; and therefore each colour pattern is a manifestation of a pattern in the process of excitation of the chromatophore motor neurons in the central nervous system.

5. In no other Cephalopod than Sepia have such colour patterns been described, and they form a complex and highly developed system of protective coloration, adapted to the conditions under which Sepia lives.

6. The types of protection afforded by the colorations are discussed. Cryptic patterns are found affording protection through obliterative shading, close environmental resemblance, and striking ruptive patterns.

7. Concealment is made more effective by the animal's ability to change its coloration from one pattern to another in times as short as two-thirds of a second.

8. Certain of the colour changes probably protect Sepia by virtue of their great rapidity for, like any sudden change in the visual field, they produce a flight reaction in the predator.

9. Such rapid change of colour thus forms a new category in the classification of the means by which colour can afford protection to animals.