Fig. 116. Oil float of Nanomia; greatly magnified.
By the time all these individuals have been added along the length of the stem, the stem itself has grown to be about three inches long ([Fig. 115]), though the tentacles hanging from the various members of the community give to the whole an appearance of much greater length. The motion of this little string of living beings is most graceful. The oil bubble ([Fig. 116]) at the upper end is their float; the swimming bells immediately below it ([Fig. 110]), by the convulsive contractions of which they move along, are their oars. The water is not taken in and expelled again by all the bells at once, but first from all the bells on one side, beginning at the lower one, and then from all those on the opposite side, beginning also at the lower one; this alternate action gives to their movements a swinging, swaying character, expressive of the utmost freedom and grace. Whether such a little community darts with a lightning-like speed through the water, or floats quietly up and down, for its movements are both rapid and gentle, it always sways in this way from side to side. Its beauty is increased by the spots of bright red scattered along the length of the stock at the base and tips of the Hydræ, as well as upon the tentacles. The movements and attitudes of the tentacles are most various. Sometimes they shoot them out in straight lines on either side, and then the aspect of the whole thing reminds one of a tiny chandelier in which the coral drops make the pendants, or they may be caught up in a succession of loops or floating in long streamers; indeed, there is no end to the fantastic forms they assume, ever astonishing you by some new combination of curves. The prevailing hue of the whole community is rosy, with the exception of the oil bubble or float, which looks a bright garnet color when seen in certain lights.
Let us now compare one of the Hydræ hanging from the stem ([Fig. 113]) with the Hybocodon ([Fig. 102]). The reader will remember the unsymmetrical bell of this singular Medusa, one half of its disk more largely developed than the other, with the proboscis hanging from the centre, and the cluster of tentacles from one side. Let us now split the bell so as to divide it in two halves with the proboscis hanging between them; next enlarge the side where there are no tentacles, and give it a triangular outline; then contract the opposite side so as to draw up the cluster of tentacles to meet the base of the proboscis, and what have we? The proboscis now corresponds to the Hydra of our Nanomia, with the cluster of tentacles attached to its upper edge ([Fig. 113]), while the enlarged half of the bell represents the shield. If this homology be correct it shows that the Nanomia is not, as some naturalists have supposed all the Siphonophores to be, a single animal, its different parts being a mere collection of organs endowed with special functions, as feeding, locomotion, reproduction, &c., but that it is indeed a community of distinct individuals corresponding exactly to the polymorphous Hydroids, whose stocks are attached, such as Hydractinia, and differing from them only in being free and floating.
Fig. 117. Physalia; a b air sac with crest c, m bunches of individuals, n central tentacles, t t expanded tentacles. (Agassiz.)
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.
Physalia. (Physalia Arethusa Til.)