_b. Physophoridæ._ These creatures are distinguished by the presence of a peculiar organ, the float, or _pneumatophore_, which is a sac enclosing a smaller sac. The float is formed by a reflexion of both the ectoderm and endoderm, and serves to buoy up the animal at the surface of the sea. The best known species is the Physalia, or Portuguese Man-o'-War. 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, coenosarc 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. "The ridge of the crest is tipped with lake, and the pointed end is stained deep bluish-green about the aperture of the pneumatocyst. "The bases of the tentacles are deep blue; the polypites deep blue at their bases, and frequently bright yellow at their apices; the velvetty masses of reproductive organs and buds on the under surface are light green." He further remarks that the tentacles have reniform thickenings at regular intervals, and "the substance of each thickening has a dark blue colour, and imbedded within it are myriads of close-set, colourless, spherical thread-cells."