Almost the whole of the body of the Physalia is filled by an air-vessel, so that it floats on the surface of the sea, and is wafted to and fro by the wind. The bladder containing the air is enclosed in two membranes, the outer one dense, thick, and elastic, the inner formed of delicate fibres and lined with cilia. The air-sac is only attached to one part of the interior; and there it communicates with the exterior by a small aperture, which may be seen at about half an inch from the pointed apex of the animal. The body is several inches long, of a delicate pale green colour, passing gradually into dark indigo blue on the under-surface; the ridge of the crest is tipped with dark crimson, and the pointed end is stained with deep bluish green. [Illustration: Fig. 121. The Physalia.] The appendages, which hang down from the inferior and thick part of the body, are large and small branchless tentacles of various lengths, and sterile polypites in different stages of development. In some individuals the tentacles are nine or ten feet long, of a deep blue colour at their origin, and formed of two distinct parts, which have a common base. One is a long conical bag, formed by an extension of the under-surface of the body lined with cilia, and ending in a pointed apex full of stinging thread-cells. It is flat on one side, attached throughout its length to the tentacle, and is supposed to furnish poison for the stings. The tentacle itself is a closed tube whose canal communicates with the cavity of the long sac, and consequently with that in the animal’s body. The interior of the tentacle is ciliated, its upper part is gathered into folds; and the rest, which hangs straight down, is like a delicate narrow ribbon, highly contractile from muscular fibres, of which the most conspicuous are longitudinal. The tentacle is marked at regular intervals by blue kidney-shaped masses, containing myriads of powerful thread-cells, in which the threads of the darts are coiled in a spiral, and contain muscular fibres, that serve to contract and extend them. The smaller tentacles vary as much in length as the large ones; they are of similar structure, but of a paler colour, and are indiscriminately mixed with the other appendages. The polypites, which are direct processes from the under-surface of the body, are crowded in groups of various sizes round the base of the large tentacles and mixed with the small; they are of a deep blue at their base, frequently of a bright yellow at their extremities, and on an average about three-fourths of an inch long. They are as irritable and contractile as the tentacles, and are in constant motion. Their mouth is large, with an everted lip armed with thread-cells; it sucks in the prey caught and brought to it by the contraction of the tentacles, and which is speedily dissolved by the powerful solvent juices in its digesting chamber.