On Molecular and Microscopic Science, Volume 2 (of 2)

by Mary Somerville

The Physophora hydrostatica ([fig. 120]), common in the Mediterranean, has a transparent pear-shaped air-vessel tipped with red, from which the slender cord-like tube of the body descends. Immediately below the air-vessel, a number of buds and young bells are attached, followed by a series of perfect three-lobed swimming bells, placed on each side obliquely one below the other; and as they alternate and embrace the body with their deeply excavated sides, they give it the appearance of a crystal cone. Four canals spring from the hollow stalks of the bells, traverse them, and end in a circular canal close to the membranaceous iris which surrounds the margin of the internal cavity. Below the cone the tubular body expands, and is twisted into a flat spiral, so as to form a hollow disk or bulb, to which three different circlets of organs are appended. The first and uppermost is a coronet of red, worm-like, closed sacs, in constant motion, with large thread-cells at their pointed extremities. They are attached to the upper surface of the bulb by their broad bases, and communicate with its tubes by a small valve. Male and female capsules follow either in a circle, or mixed with the third and undermost circlet of organs, which consist of sterile nourishing polypites, fixed by hollow stems to the bulb, each of which has a long branching tentacle fixed to the base of its digesting cavity.

There are as many polypites on the under-side of the bulb as there are red worm-like sacs on its upper edge. Each polypite consists of three distinct parts. The posterior part is a hollow red stalk inserted under the circumference of the disk; the second part is a bright yellow globular expansion containing the digestive cavity lined with cilia; the third and anterior part, which ends with the mouth, is quite colourless and transparent, and assumes various shapes by constant expansion and contraction.

At the limit between the red stalk and the yellow globular part of the polypites there is a tuft of cylindrical appendages, from which a long tentacle descends with its secondary tentacles and red nettle-bulbs. All the canals of this Physophora are connected, and their walls are lined with muscular fibres, either circular, longitudinal, or both, which give a marvellous contractile and motive power. When the animal is suspended from the surface of the sea by its float, every member is in motion, especially the numerous tentacles, which are perpetually in search of food, and are so extremely sensitive that even a sudden motion of the water makes them shrink under the red worm-like organs on the edge of the disk. This animal is generally from one to three inches long.

All the preceding members of the physograde group are really campanograde, for the action of the wind upon the floats of the Physophoridæ must be small, otherwise they would not be furnished with so many swimming cups. The Physaliidæ and Velellidæ are the only two orders that are truly physograde, for the wind is their only locomotive power.

The Physalia, or ‘Spanish man-of-war’ of sailors, is by far the most formidable animal of the Acalephæ tribe; its poisonous stings, which burn like fire, inflict instant death on the inferior animals, and give painful wounds to man himself. Its body, as it floats, is a long horizontal double sac ([fig. 121]), which begins with a blunt point, gradually enlarges, and becomes cylindrical about the middle; then it somewhat suddenly widens in a transverse or lateral direction. Along the upper surface of the pointed half the membrane or wall of the sac is raised into a transversely placed crest, which dies away at the enlarged end. The greater part of the body is smooth, but the under-surface of the transversely enlarged end swells into lobes, from whence numerous tentacles and other organs descend.

Almost the whole of the body of the Physalia is filled by an air-vessel, so that it floats on the surface of the sea, and is wafted to and fro by the wind. The bladder containing the air is enclosed in two membranes, the outer one dense, thick, and elastic, the inner formed of delicate fibres and lined with cilia. The air-sac is only attached to one part of the interior; and there it communicates with the exterior by a small aperture, which may be seen at about half an inch from the pointed apex of the animal. The body is several inches long, of a delicate pale green colour, passing gradually into dark indigo blue on the under-surface; the ridge of the crest is tipped with dark crimson, and the pointed end is stained with deep bluish green.

Fig. 121. The Physalia.

The appendages, which hang down from the inferior and thick part of the body, are large and small branchless tentacles of various lengths, and sterile polypites in different stages of development. In some individuals the tentacles are nine or ten feet long, of a deep blue colour at their origin, and formed of two distinct parts, which have a common base. One is a long conical bag, formed by an extension of the under-surface of the body lined with cilia, and ending in a pointed apex full of stinging thread-cells. It is flat on one side, attached throughout its length to the tentacle, and is supposed to furnish poison for the stings. The tentacle itself is a closed tube whose canal communicates with the cavity of the long sac, and consequently with that in the animal’s body. The interior of the tentacle is ciliated, its upper part is gathered into folds; and the rest, which hangs straight down, is like a delicate narrow ribbon, highly contractile from muscular fibres, of which the most conspicuous are longitudinal. The tentacle is marked at regular intervals by blue kidney-shaped masses, containing myriads of powerful thread-cells, in which the threads of the darts are coiled in a spiral, and contain muscular fibres, that serve to contract and extend them. The smaller tentacles vary as much in length as the large ones; they are of similar structure, but of a paler colour, and are indiscriminately mixed with the other appendages.

The polypites, which are direct processes from the under-surface of the body, are crowded in groups of various sizes round the base of the large tentacles and mixed with the small; they are of a deep blue at their base, frequently of a bright yellow at their extremities, and on an average about three-fourths of an inch long. They are as irritable and contractile as the tentacles, and are in constant motion. Their mouth is large, with an everted lip armed with thread-cells; it sucks in the prey caught and brought to it by the contraction of the tentacles, and which is speedily dissolved by the powerful solvent juices in its digesting chamber.