Prof. Huxley, in his monograph on the Oceanic Hydrozoa, gives many details of the colouration; and, not having had much opportunity of studying them, the following observations are taken from his work. It will be seen that the Physophoridæ illustrate the structural distribution of colour in a remarkable manner.
Stephanomia amphitridis, the hydrophyllia, colourless, and so transparent as to be almost imperceptible in water, cœnosarc whitish, enlarged portions of polypites, pink or scarlet, sacs of tentacles scarlet.
The enlarged portion of the polypites is marked with red striæ, "which are simply elevations of the endoderm, containing thread-cells and coloured granules." The small polypites do not possess these elevations, and are colourless.
Agalma breve, like a prismatic mass of crystal, with pink float and polypites.
Athorybia rosacea, float pink, with radiating dark-brown striæ, made up of dots; polypites lightish red, shading to pink at their apices; tentacles yellowish or colourless, with dark-brown sacculi; thread-cells dark brown.
Rhizophysa filiformis, pink, with deep red patch surrounding the aperture of the pneumatocyst.
Physalia caravilla, bright purplish-red, with dark extremities, and blue lines in the folds of the crest; polypites violet, with whitish points, larger tentacles red, with dark purple acetabula, smaller tentacles blue, bundles of buds reddish.
P. pelagica, in young individuals pale blue, in adult both ends green, with highest part of crest purple, tentacles blue, with dark acetabula; polypites dark blue, with yellow points.
P. utriculus. Prof. Huxley describes a specimen doubtfully referred to this species very fully, as follows:—
"The general colour of the hydrosoma is a pale, delicate green, passing gradually into a dark, indigo blue, on the under surface.