On shells and stones, on sea-weeds or on floating logs, there may often be observed a growth of exquisitely delicate branches, looking at first sight more like a small bunch of moss than anything else. But gather such a mossy tuft and place it in a glass bowl filled with sea-water, and you will presently find that it is full of life and activity. Every branch of this miniature shrub terminates in a little club-shaped head, upon which are scattered a number of tentacles. They are in constant motion, extending and contracting their tentacles, some of the heads stretched upwards, others bent downwards, all seeming very busy and active. Each tentacle has a globular tip filled with a multitude of cells, the so-called lasso-cells, each one of which conceals a coiled-up thread. These organs serve to seize the prey, shooting out their long threads, thus entangling the victim in a net more delicate than the finest spider's web, and then carrying it to the mouth by the aid of the lower part of the tentacle. The complication of structure in these animals, a whole community of which, numbering from twenty to thirty individuals, is not more than an inch in height, is truly wonderful. In such a community the different animals are hardly larger than a good-sized pin's head; and yet every individual has a digestive cavity and a complete system of circulation. Its body consists of a cavity inclosed in a double wall, continuing along the whole length of each branch till it joins the common stem forming the base of the stock. In this cavity the food becomes softened and liquefied by the water that enters with it through the mouth, and is thus transformed into a circulating fluid which flows from each head to the very base of the community and back again. The inner surface of the digestive cavity is lined with brownish-red granules, which probably aid in the process of digestion; they frequently become loosened, fall into the circulating fluid, and may be seen borne along the stream as it passes up and down. The rosy tint of the little community is due to these reddish granules.
This crowd of beings united in a common life began as one such little Hydra-like animal as I have described above,—floating free at first, then becoming attached, and growing into a populous stock by putting out buds at different heights along the length of the stem. The formation of such a bud is very simple, produced by the folding outwardly of the double wall of the body, appearing first as a slight projection of the stem sideways, which elongates gradually, putting out tentacles as it grows longer, while at the upper end an aperture is formed to make the mouth. This is one of the lower group of Radiates, known as Hydroids, and long believed to be Polyps, from their mode of living in communities and reproducing their kind by budding, after the fashion of Corals. But if such a little tuft of Hydroids has been gathered in spring, a close observer may have an opportunity of watching the growth of another kind of individual from it, which would seem to show its alliance with the Acalephs rather than the Polyps. At any time late in February or early in March, bulb-like projections, more globular than the somewhat elongated buds of the true Hydroid heads, may be seen growing either among the tentacles of one of these little animals, or just below the head where it merges in the stem,[3] Very delicate and transparent in substance, it is hardly perceptible at first; and the gradual formation of its internal structure is the less easily discerned, because a horny sheath, forming the outer covering of the Hydroid stock, extends to inclose and shield the new-comer, whom we shall see to be so different from the animal that gives it birth that one would suppose the Hydroid parent must be as much surprised at the sight of its offspring as the Hen that has accidentally hatched a Duck's egg. At the right moment this film is torn open by the convulsive contractions of the animal, which, thus freed from its envelope, begins at once to expand. By this time this little bud has assumed the form of a Medusoid or Jelly-Fish disk, with its four tubes radiating from the central cavity. The proboscis, so characteristic of all Jelly-Fishes, hangs from the central opening; and the tentacles, coiled within the internal cavity up to this time, now make their appearance, and we have a complete little Medusa growing upon the Hydroid head. Gradually the point by which it is attached to the parent-stock narrows and becomes more and more contracted, till the animal drops off and swims away, a free Jelly-Fish.
The substance of these animals seems to have hardly more density or solidity than their native element. I remember showing one to a friend who had never seen such an animal before, and after watching its graceful motions for a moment in the glass bowl where it was swimming, he asked, "Is it anything more than organized water?" The question was very descriptive; for so little did it seem to differ in substance from the water in which it floated that one might well fancy that some drops had taken upon themselves organic structure, and had begun to live and move. It swims by means of rapid contractions and expansions of its disk, thus impelling itself through the water, its tentacles floating behind it and measuring many times the length of the body. The disk is very convex, as will be seen by the wood-cut; four tubes radiate from the central cavity to the periphery, where they unite in a circular tube around the margin and connect also with the four tentacles; from the centre of the lower surface hangs the proboscis, terminating in a mouth. Notwithstanding the delicate structure of this little being, it is exceedingly voracious. It places itself upon the surface of the animal on which it feeds, and, if it have any hard parts, it simply sucks the juices, dropping the dead carcass immediately after; but it swallows whole the little Acalephs of other Species and other soft animals that come in its way. Early in summer these Jelly-Fishes drop their eggs, little transparent pear-shaped bodies, covered with vibratile cilia. They swim about for a time, until they have found a resting-place, where they attach themselves, each one founding a Hydroid stock of its own, which will in time produce a new brood of Medusae.
This series of facts, presented here in their connection, had been observed separately before their true relation was understood. Investigations had been made on the Hydroid stock, described as Coryne, and upon its Medusoid offspring, described as Sarsia, named after the naturalist Sars, whose beautiful papers upon this class of animals have associated his name with it; but the investigations by which all these facts have been associated in one connected series are very recent. These transformations do not correspond to our common idea of metamorphoses, as observed in the Insect, for instance. In the Butterfly's life we have always one and the same individual,—the Caterpillar passing into the Chrysalis state, and the Chrysalis passing into the condition of the Winged Insect. But in the case I have been describing, while the Hydroid gives birth to the Medusa, it still preserves its own distinct existence; and the different forms developed on one stock seem to be two parallel lives, and not the various phases of one and the same life. This group of Hydroids retains the name of Coryne; and the Medusa born from it, Sarsia, has received, as I have said, the name of the distinguished investigator to whose labors we owe much of our present knowledge of these animals.—Let us look now at another group of Hydroids, whose mode of development is equally curious and interesting.
The little transparent embryos from which they arise, oval in form, with a slight, scarcely perceptible depression at one end, resemble the embryos of Coryne already described. They may be seen in great numbers in the spring, floating about in the water, or rather swimming,—for the motion of all Radiates in their earliest stage of existence is rapid and constant, in consequence of the vibratile cilia that cover the surface. At this stage of its existence such an embryo is perfectly free, but presently its wandering life comes to an end; it shows a disposition to become fixed, and proceeds to choose a suitable resting-place. I use the word "choose" advisedly; for though at this time the little embryo seems to have no developed organs, it yet exercises a certain discrimination in its selection of a home. Slightly pear-shaped in form, it settles down upon its narrower end; it wavers and sways to and fro, as if trying to get a firm foothold and force itself down upon the surface to which it adheres; but presently, as if dissatisfied with the spot it has chosen, it suddenly breaks loose and swims away to another locality, where the same examination is repeated, not more to its own satisfaction apparently, for the creature will renew the experiment half a dozen times, perhaps, before making a final selection and becoming permanently attached to the soil. In the course of this process the lower end becomes flattened, and moulds itself to the shape of the body on which it rests. Once settled, this animal, thus far hardly more than a transparent oblong body without any distinct organs, begins to develop rapidly. It elongates, forming a kind of cup-like base or stem, the upper end spreads somewhat, the depression at its centre deepens, a mouth is formed that gapes widely and opens into the digestive cavity, and the upper margin spreads out to form a number of tentacles, few at first, but growing more and more numerous till a wreath is completed all around it. In this condition the young Jelly-Fish has been described under the name of Scyphostoma. As soon as this wreath of tentacles is formed, a constriction takes place below it, thus separating the upper portion of the animal from the lower by a marked dividing-line. Presently a second constriction takes place below the first, then a third, till the entire length of the animal is divided across by a number of such transverse constrictions, the whole body growing, meanwhile, in height. But now an extraordinary change takes place in the portions thus divided off. Each one assumes a distinct organic structure, as if it had an individual life of its own. The margin becomes lobed in eight deep scallops, and a tube or canal runs through the centre of each such lobe to the centre of the body, where a digestive cavity is already formed. At this time the constrictions have deepened, so that the margins of all the successive divisions of the little Hydroid are very prominent, and the whole animal looks like a pile of saucers, or of disks with scalloped edges and the convex side turned downward. Its general aspect may be compared to a string of Lilac-blossoms, such as the children make for necklaces in the spring, in which the base of one blossom is inserted into the upper side of the one below it. In this condition our Jelly-Fish has been called Strobila.
