“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.”
VELELLA.
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]
STEPHANOMIA.
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.
The long tube or common stem is very irritable, and under stimulus is coiled up in a spiral, then gradually relaxed, and allowed to hang loosely down. Attached to it throughout are the polypites, or stomach-mouths, protected by leaf-like organs, which overlap them. Numerous tentacles, too, of exceeding length and tenuity, and throwing off branches, spring from the stem throughout its length, all the extremities of each forming tiny oval knobs of spiral coils, the whole capable of being thrown into the most beautiful and graceful convolutions, now bent at right angles, and again thrown out in a series of light airy-looking arches.
The exquisitely beautiful specimen described was taken in sultry weather, when the sea was perfectly calm. “It lived in a large globe of sea-water for several days, and was a most striking and beautiful object: it generally lay quite upright in the water, the slightest ripple upon the surface of which set it in motion; and immediately coiling up its fishing lines, and shortening the length of its stolon by about one-third, it threw its set of little balloons into strong pulsations, until, weary of descending, it submitted to be guided by its brightly glittering head, and soon ascended to the surface. When subjected to examination it soon died, and was not long until it had completely broken itself up.”[127]
SARSIA.
The nectocalyx or swimming-bell is, however, seen in a much more developed condition in the delicate forms which swarm in our harbours and about our rocky coasts in summer and early autumn, and are known by the name of Naked-eyed Medusæ. A common and very characteristic example of this order is the Tongued Sarsia,[128] in which a single swimming-bell forms the greatest portion of the whole creature. It is a tall dome of crystalline colourless flesh, thick at the summit and thinning off at the edges; about half an inch in height. From the interior of this dome hangs the single polypite, exactly as the clapper hangs from the top of a bell. It is long and cylindrical, abruptly attenuated at the upper part into a sort of foot-stalk, and at the extremity enlarging into four fleshy, very protrusile, lips, capable of seizing and sucking in an object much larger than the diameter of the whole polypite. I have seen a Sarsia, in confinement in a tank, lay hold of, and swallow, a newly-hatched fish, notwithstanding the activity of the latter. For hours afterwards the little green-eyed fry was visible, the engulphment being a very slow process; the Medusa, however, never let go its hold; and gradually the tiny fish was sucked into the interior, and passed up the cavity of the polypite, becoming more and more cloudy and indistinct, as digestion in the stomach gradually dissolved its tissues.
These little creatures are endowed with very effective powers of locomotion. In the unbounded freedom of their native sea, and in the limited dimensions of a glass vase, they are alike sprightly. By rapid pump-like contractions of their nectocalyx, they dart through the water, and shoot round and round, almost with the force and swiftness of a swimming fish. The summit of the bell always goes foremost, whether the direction of the movement be vertical, horizontal, or, as is most commonly the case, oblique; and the tentacles, and the long white polypite, drag behind in trailing lines. Now and anon, the shooting is suddenly suspended, the bell hangs over and remains awhile motionless, the tentacles are allowed to depend like spiders’ webs, or are suddenly drawn up into shrivelled puckers, become mutually entangled and intertwisted, then slowly free themselves, and hang down again. Sometimes the motionless bell itself sinks very gradually, and the tentacle-threads take the most elegant curves and arches in their descent.