Among the numerous appendages attached to the under-surface of the Physalia, there are bluish-green velvety masses fixed to extremely small branching processes from the body of the animal, which seem with a microscope to consist of tentacles, polypites in various stages of development, male reproductive capsules which are never detached, and female buds that are developed into medusiform zooids, and are presumed to become free as in other cases. The Physaliidæ are social animals, assembling in numerous shoals in the warm latitudes of the Atlantic and Pacific. They naturally have their crest vertical, kept steady by their tentacles, which drag down in the water; but Professor Huxley has seen them at play, in a dead calm, tumbling over and over. The Physalia does not possess the power of emptying and refilling its float with air: it is doubted whether any of the physograde animals have that power, but the subject is still in abeyance.
The Velellidæ are little sailing members of the physograde group. The Velella spirans ([fig. 122]), a Mediterranean species, has a body or deck consisting of a hollow horizontal disk, of a firm but flexible cartilaginous substance, surrounded by a delicate membranous fringe or limb half the width of the body. A triangular vertical crest, formed of a firm transparent plate, also encompassed by a delicate limb, is fixed diagonally from one angle of the disk to the other, but not on the fringe; and as the natural position of the Velella is to float horizontally on the surface of the water, the crest is exposed to the wind and acts as a sail.
The float or air-vessel is flat, horizontal, and nearly fills the whole body of the animal: it consists of two thin, firm, and rather concave plates joined at their free edges, and united also by a number of concentric vertical partitions, between which there is a series of concentric chambers or galleries filled with air. The chambers communicate with one another by apertures in the dividing membrane; they also communicate with the exterior by perforations through the surface of the body. Very long pneumatic filaments, that is tubes filled with air, descend from the inferior surface of the float, and pass through the lower plate of the disk into the water.
Fig. 122. Velella spirans:—1, upper side; 2, under-side.
The disk is transparent, and appears to be white from the air within it; and it is marked by concentric rings corresponding to the divisions in the air-vessel below. The fringe-like limb that surrounds it is flat, flexible, semi-transparent, and of the richest dark blue passing into green, with a light blue ring; it is very contractile, and moves in slow undulations. The sail or crest is thin, firm, and transparent, covered by a bluish membrane; its limb is dark blue, crossed by waving yellow lines.
An irregular microscopic network of vascular canals, containing yellow matter, is seen in the soft substance which covers the sail; it ends in a canal round its margin. A similar system exists both in the upper and under-surface of the disk. All these systems are connected with one another, and with organs pending from the inferior side of the disk, which are hid when the Velella is in its natural horizontal position. These organs consist of a large central sterile polypite, which supplies the whole system with elaborated juices; it is surrounded by smaller polypites, which are both nutritive and reproductive; and the whole is encircled with a ring of prehensile and armed tentacles fastened to the rim of the disk, immediately adjoining to the under-side of the limb. The pneumatic filaments already mentioned are mixed with these different organs.
In the Velellidæ caught by M. Vogt, he invariably found the stomachs of the large as well as of the small polypites, full of the carapaces of minute crustacea, shells, the bones of small fishes, and larvæ, so as even to be swelled out with them. The indigestible parts are thrown out at the mouth, and the elaborated juices are transferred to the various systems of canals to be distributed through all the members of the animal. The mouths of the small polypites take various forms; sometimes they are wide and trumpet-shaped, with everted lips, sometimes they are contracted. These small polypites consist of a double sac, fastened to the disk by a hollow stem with many rounded elevations on their surface full of thread-cells. The tentacles of the Velellidæ are strong, thick, club-shaped tubes, completely closed at their extremities, which abound in thread-cells; their cavity is filled with a transparent liquid, supposed to play an important part in their elongation.
Medusiform zooids are formed on the slender stems of the small polypites. It is presumed that they lay fertilized eggs which yield Velellidæ, so that this animal has probably alternate states of existence; but nothing is known of its earliest stages of development. The youngest form yet discovered is that described by Prof. Huxley, in his excellent monograph on ‘Oceanic Hydrozoa.’[[26]] The Velellidæ are inhabitants of warm and tropical seas, but are occasionally found on the coasts of Great Britain, being carried by the Gulf Stream to the Bay of Biscay, and thence wafted northwards by the prevalent winds.
Although the Porpita, a genus of the Velellidæ, has no sail, it is akin to the Velellæ in size and structure. The body of the Porpita consists of two circular cartilaginous disks, united at their edges and surrounded by a blue membranous limb. On the surface of the upper disk there are beautifully radiating striæ, each of which ends at the circumference of the disk in a little protuberance, which gives it the appearance of a toothed wheel. A large sterile polypite occupies the centre of the under-surface of the body, surrounded by a zone full of smaller ones; and the space between the zone and the blue limb is occupied by a narrow area of a reticulated appearance, to which numerous circles of tentacles are fixed, that spread out and radiate all around the margin of the animal. The interior circular rows are simple, short, and fleshy, not extending much beyond the edge of the limb: the succeeding circles are gradually longer, while the exterior row, which extends far beyond the limb, are branched and beset with slender filaments, ending in minute globes, sometimes filled with air, so that a Porpita is like a floating daisy, though differently coloured. The Porpita glandifera, a pretty little inhabitant of the Mediterranean, which only appears in calm weather, is not more than eight lines in diameter; somewhat convex, white, marked by radiating striæ, and encompassed by a dark blue limb. The central polypite and those next to it are whitish, the rest become of a darker blue towards the limb; the tentacles are pellucid and bluish, and the three last rows have little dark blue globes attached to them by slender filaments.