ABSTRACT In the estuary of the Severn, in the neighbourhood of Aust and the New Passage, there are to be found, thriving in the muddy water, large numbers of Actinias, principally of two common species, Actinia mesembryanthemum and Bunodes crassicornis; and farther down the channel, at Weston-super-Mare, Bunodes crassicornis is to be found in great abundance. As might be expected from the nontransparency of the medium in which they live the colours of these anemonies are not by any means so bright as are those of individuals which live in pure sea-water. The red colouring seems to be especially affected. Actinia mesembryanthemum in this muddy water assumes a pale olive, or often a mere dirty-white colour, and Bunodes crassicornis a transparent green tint, which is especially remarkable in Weston specimens. About Aust Cliffs I found in one especial locality, however, some specimens of Bunodes crassicornis, which retained a vivid red colouring, and were almost as bright as marine specimens. The reason appeared to be that they were attached to the bottom of a rocky channel, through which, as the tide fell, there drained off in a continuous stream water with which a large natural rock basin, standing at a slightly higher level, had been filled at high water. The dense charge of mud held in suspension by the Severn water when it is in motion was precipitated rapidly when this water came to rest in the still rock pool. Thus, a short time after the stream had begun to flow from the pool, the water of the stream became perfectly clear and transparent, and the specimens of Crassicornis living in it, by the help of the greater quantity and different quality of the light they received, were able to maintain their red colouring, though their congeners close by were almost colourless or merely greenish. On examining the red colouring matter of one of these red specimens with the spectroscope, I found that that which I scraped off from the general body-walls and tentacles gave no absorption-band, whilst that from the disc surrounding the mouth did give a single well-marked one, nearly coinciding with the less refrangible band of hæmoglobin. I determined the position of the band accurately at the time I was working at the subject, and showed it to my friend Mr. E. R. Lankester, but unfortunately the drawing was mislaid, and in the hurry consequent on preparing for the Circumnavigation expedition, I have been unable to make a re-examination. I think, however, that there is sufficient interest in the observation, though imperfect, to warrant its publication. Mr. Lankester suggested to me the term Actiniochrome for the red colouring matter giving this single absorption-band. I was unable to obtain the matter in solution, though I tried all the ordinary solvents of animal colouring matters.