The homologies of the Siphonophoræ or floating Hydroids, with many of the fixed Hydroids, is perhaps more striking when we compare the earlier stages of their growth. Suppose, for instance, that the planula of our Melicertum (See Fig. 81) should undergo its development without becoming attached to the ground,--what should we then have? A floating community (Fig. 83), including on the same stock like the Nanomia, both sterile and fertile Hydræ, from the latter of which Medusæ bells are developed. The little Hydractinia community (Fig. 100), in which we have no less than four distinct kinds of individuals, each performing a definite distinct function, affords a still better comparison.
[Illustration: Fig. 117. Physalia; _a_ _b_ air sac with crest _c_, _m_ bunches of individuals, _n_ central tentacles, _t_ _t_ expanded tentacles. (_Agassiz_.)]
Among the most beautiful of the Siphonophores, is the well-known Physalia or Portuguese man-of-war, represented in Fig. 117. The float above is a sort of crested sac or bladder, while the long streamers below consist of a number of individuals corresponding in their nature and functions to those composing a Hydroid community. Among them are the fertile and sterile Hydræ (Fig. 118), the feeders and Medusæ bells (Fig. 119). The Physalia properly belongs to tropical waters, but sometimes floats northward, in the warm current of the Gulf Stream, and is stranded on Cape Cod. When found so far from their home, however, they have usually lost much of their vividness of color; to judge of their beauty one should see them in the Gulf of Mexico, sailing along with their brilliant float fully expanded, their crest raised, and their long tentacles trailing after them.
[Illustration: Fig. 118. Bunch of Hydræ; _a_ base of attachment, _b_ _b_ _b_ single Hydræ, _c_ _c_ tentacles. (_Agassiz_.)]
[Illustration: Fig. 119. Bunch of Hydræ; cluster of Medusæ; _b_ _b_ Hydræ with tentacles, _c_ _d_ bunches of Medusæ (_Agassiz_.)]
Another very beautiful floating Hydroid, occasionally caught in our waters, though its home is also far to the south, is the Velella (Fig. 120). It is bright blue in color, and in form not unlike a little flat boat with an upright sail. Its Medusa (Fig. 121) resembles so much that of some of our Tubularians, that it has actually been removed on this account from the old group of Siphonophoræ, and placed next the Tubularians; another evidence of the close affinity between the former and the Hydroids.