Mr. Hughes in his account mentioned a circumstance as normal, which, being unknown to me, excited my curiosity. His specimen was accompanied by “its attendant satellites, two _Velellæ_.” In reply to my inquiries my friend gives me the following information:--“My authority for the association of Velella with Physalia is Jenkins, the collector at Tenby, who was attending me when they were found.
“The Physalia was taken by me first; and, while I was admiring it, I noticed that Jenkins continued searching for something. Immediately afterwards he came up with the Velella in his hand, at the same time stating they were generally found with the Portuguese man-of-war. As I had found the man very honest and truthful in his dealings with me (and not previously being familiar with either of the creatures), I accepted the information as correct.”
The _Velella_ is a creature closely allied to the _Physalia_, having the same essential structure, but differing greatly in form and appearance. It consists of a flat disk of thin cartilaginous shell, of a long rectangular outline, on which stands erect another similar plate somewhat triangular, extending across from corner to corner. One large polypite or mouth hangs down from the centre of the under side, and short tentacles project on all sides from the edges of the horizontal plate. The internal structure is very complex. The whole is enclosed in a thin layer of flesh, which is tinged of a fine blue colour, sometimes varied with purple and green. It is generally about two inches long, and the erect plate rather less in height. As the little creature floats on the waves, this plate forms a sail, and the breeze playing upon it, imparts to the whole animal, according to Mr. Bennett, a rotatory motion.[126]
Both these oceanic forms of the class _Hydrozoa_ agree in being quite unprovided with any apparatus for locomotion. Though their parts can be moved among themselves, and sometimes with much vigour, as we have seen in the case of the tentacles of the _Physalia_, yet the whole organism would be absolutely confined for life to one spot, were the elements motionless around it: it is merely passively driven hither and thither by the winds and waves. But there are other members of the class, and even some belonging to the same order, which have express organs of locomotion. A most exquisite example is found in _Stephanomia_, of which a specimen was taken in Kingstown harbour in July 1856, by Mr. Joseph Greene, and described and figured in the “Proceedings of the Dublin University Zoological Association” for that year. The float (_pneumatocyst_) is a small bladder, which is filled with air, and which has the appearance, when alive, of a globule of quicksilver; while the slight inequalities of its surface reflect the light in such a manner that it often looks as if a fine network of crimson veins ramified over its surface. From the end of this oval bladder a long fleshy tube hangs down in the water to the distance of six or seven inches, the upper part of which is surrounded by a number--seven to ten--of little clear transparent bells, looking very much like the blossoms of our common Arbutus, and crowded, like flowers with short footstalks, on the stem. These swimming bells (_nectocalyces_) are true organs of locomotion; for by their contraction they drive out the water contained in their cavities, and by their combined impulse, all acting together, shoot the whole creature forward. This action I will explain more in detail presently.