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

The Galeolaria lutea (fig. 118, frontispiece) is similar to the Praya diphys in having a slender, tubular body, with groups of sterile polypites and their appendages hanging at intervals along its under-side like a fringe, and also in having two swimming-bells at its anterior extremity; but it has no special small bells. The large ones differ from each other in size, form, and position. The largest is nearly cylindrical, its mouth is turned upwards, and its rim is elevated at one part into two stiff organs like the blinkers that are put over horses’ eyes: besides these, it has six salient points, which nearly close the mouth of the bell at each contraction of the muscular iris that lines the margin of the cavity. The small bell, which goes first in swimming, is thicker and shorter, and its side rises in a hump, upon which the closed end of the large bell rests, and in the cavity between the two the anterior extremity of the filiform body is fixed. Each of the groups of polypites, with their tentacles, lies immediately under its spathe-shaped protecting plate. The polypites are very contractile, and on their protuberant part, containing the digestive cavity, there is a large circular space, which, as well as the whole tissue of the polypite and the stinging capsules at the extremities of the tentacles, are of an orange colour, and are akin in structure to those described.

When very young the Galeolaria and its congeners have only their swimming-bells and one polypite group affixed to the end of a short tubular axis; by and by a second group is developed from buds between the bells and the first group; then a third is developed between the bells and the second group, and so on; the length of the body and the number of groups continue to increase indefinitely. It is only when the animal is full grown and complete in all its parts, that reproductive organs are developed towards its posterior end. Buds then appear upon the hollow stems of the polypites towards the posterior end of the body. But as the Galeolaria is diœcious, male and female buds are never on the same individual. The female buds become medusiform zooids, like those of the Praya diphys, only the transparent cup, with which it swims away from its parent, has two projections like ears on its rim.

The development of the buds in the male Galeolaria is similar. At first they are pale, but they assume an orange red colour as they advance towards maturity, and, when complete, the sac which hangs down from the centre of the transparent cup becomes of a brilliant vermilion. These male and female medusa-zooids swim about for several days, and the fertilized eggs are hatched into young Galeolariæ, male and female.

Thus the Galeolaria lutea has two kinds of polypites, both nutritive, but one is sterile, the other prolific. The latter are similar to the prolific individuals of the syncorine Hydræ, in which the anterior part is a digestive organ, while on the base or stalk true medusa-zooids are found. It is curious that the spathe-protecting plate of the Galeolaria appears in the egg as a globe of such size that the other parts seem to be merely the appendages.

The Apolemia contorta (fig. 119) unites the most graceful form to the utmost transparency and delicacy of tissue. It has a double float, the first small and globular, the second long and oval. The neck is short, the rose-coloured body is flat as a ribbon, and covered with thin, curved, pointed, and imbricated plates, like tiles on the roof of a house, but so minute that they are only perceptible to the naked eye by a slight iridescence. At the extremity of the short neck buds, semi-developed buds, and perfect swimming cups are arranged in vertical series; and as the flat body is twisted into a spiral to its farthest end, the cluster of bells forms a perfect cone with the float at its apex. The bells are flattened; and there is always in their more solid posterior part a single canal rising directly from the general trunk which divides into four branches; and these, having traversed the swimming cavity, unite anew in a circular canal, or iris, destined to shut and open the cup.

The sterile polypites that are attached at intervals by their hollow stems to the twisted body of the Apolemia, have twelve rows of cells inserted in the bright lining of their digestive cavities; their anterior part has a trumpet-shaped mouth full of thread-cells. The tentacles affixed to their stems and their secondary lines are like those of the Diphyidæ. Besides these sterile polypites, which serve only to feed the animal, the Apolemia has a kind of mouthless prolific organs, which do not contribute to the general nourishment: each group has a pair of them attached to the extremity of a branching stem. They resemble polypites in being long and contractile at their extremities; the interior is full of a substance like sarcode, and encompassed by a red ring. Female buds yielding eggs appear on the stem of one of these organs, while male buds are developed into medusiform zooids on the stem of the other, which become detached, swim away, and the fertilized eggs yield young Apolemiæ.

The natural position of an individual of the family of the Physophoridæ when at rest is to hang perpendicularly from its air-vessel. The body, which begins with a pyriform float, descends in a slender filiform scarlet tube with a number of hyaline natatory cups or bells attached on each side. The lower end of the body enlarges into a bulb or disk supporting various appendages.

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.

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.

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.

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.

The Porpita has a horizontal air-vessel divided vertically into air-chambers like the Velella, but they are much more numerous. In a middle-sized Porpita, four or five lines in diameter, there are twenty-three or twenty-four air-chambers surrounding a central one, and eighty or ninety pneumatic filaments, so that the animal is extremely buoyant. Brown matter, supposed to be a liver, lies directly below the undermost wall of the air-vessel, through which, as well as through the base of the animal, all the pneumatic filaments penetrate; the greater number go straight down into the water, but a portion of them terminate in the walls of the polypites.

A complete system of canals, ciliated internally, traverses all parts of the animal; and it may be presumed that the cilia maintain its juices in a state of circulation similar to that in the Velella; and the functions of the polypites, great and small, that are in connection with the liver, are also similar to those of the Velella. The Porpita is armed with thread-cells like all the class. The central polypite is sterile and nutritive; the small ones are both nutritive and reproductive: buds spring from their stems, which become independent male and female medusiform zooids, swim away from their parent and produce abundance of eggs, whence a new generation of Porpitæ arise.

In this singular class of fresh-water and oceanic Hydrozoa, the internal cilia, aided by the contraction of the walls of the body, are the sole means provided by nature for the circulation of the fluids.