The life of the Jelly-fishes, with the exception of the Millepores and the like, is short in comparison to that of other Radiates. While Polyps live for many years, and Star-fishes and Sea-urchins require ten or fifteen years to attain their full size, the short existence of the Acaleph, with all its changes, is accomplished in one year. The breeding season being in the autumn, the egg grows into a Hydroid during the winter; in the spring the Jelly-fish is freed from the Hydroid stock, or developed upon it as the case may be; it attains its full size in the fall, lays its eggs and dies, and the cycle is complete. The autumn storms make fearful havoc among them, swarms of them being killed by the fall rains, after which they may be found thrown up on the beaches in great numbers. When we consider the size of these Jelly-fishes, their rapidity of growth seems very remarkable. Our common Aurelia measures some twelve to eighteen inches in diameter when full grown, and yet in the winter it is a Hydra so small as almost to escape notice. Still more striking is the rapid increase of our Cyanea, that giant among Jelly-fishes, which, were it not for the soft, gelatinous consistency of its body, would be one of the most formidable among our marine animals.
Before entering upon the descriptions of the special kinds of Jelly-fishes, we would remind our readers that the radiate plan of structure is reproduced in this class of animals as distinctly as in the Polyps, though under a different aspect. Here also we find that there is a central digestive cavity from which all the radiating cavities, whether simple or ramified, diverge toward the periphery. It is true that the open chambers of the Polyps are here transformed into narrow tubes, by the thickening of the dividing partitions; or in other words, the open spaces of the Polyps correspond to tubes in the Acalephs, while the partitions in the Polyps correspond to the thick masses of the body dividing the tubes in the Acalephs. But the principle of radiation on which the whole branch of Radiates is constructed controls the organization of Acalephs no less than that of the other classes, so that a transverse section across any Polyp ([Fig. 1]), or across any Acaleph ([Fig. 50]), or across any Echinoderm (Fig. 140), shows their internal structure to be based upon a radiation of all parts from the centre to the periphery.
That there may be no vagueness as to the terms used hereafter, we would add one word respecting the nomenclature of this class, whose aliases might baffle the sagacity of a police detective. The names Acalephs, Medusæ, or the more common appellation of Jelly-fishes, cover the same ground, and are applied indiscriminately to the animals they represent. The name Jelly-fish is an inappropriate one, though the gelatinous consistency of these animals is accurately enough expressed by it; but they have no more structural relation to a fish than to a bird or an insect. They have, however, received this name before the structure of animals was understood, when all animals inhabiting the waters were indiscriminately called fishes, and it is now in such general use that it would be difficult to change it. The name Medusa is derived from their long tentacular appendages, sometimes wound up in a close coil, sometimes thrown out to a great distance, sometimes but half unfolded, and aptly enough compared to the snaky locks of Medusa. Their third and oldest appellation, that of Acalephs,—alluding to their stinging or nettling property, and given to them and like animals by Aristotle, in the first instance, but afterwards applied by Cuvier in a more limited sense to Jelly-fishes,—is the most generally accepted, and perhaps the most appropriate of all.
The subject of nomenclature is not altogether so dry and arid as it seems to many who do not fully understand the significance of scientific names. Not only do they often express with terse precision the character of the animal or plant they signify, but there is also no little sentiment concealed under these jaw-breaking appellations. As seafaring men call their vessels after friends or sweethearts, or commemorate in this way some impressive event, or some object of their reverence, so have naturalists, under their fabrication of appropriate names, veiled many a graceful allusion, either to the great leaders of our science, or to some more intimate personal affection. The Linnæa borealis was well named after his famous master, by a disciple of the great Norwegian naturalist; Goethea semperflorens, the ever-blooming, is another tribute of the same kind, while the pretty, graceful little Lizzia, named by Forbes, is one instance among many of a more affectionate reference to nearer friends. The allusions of this kind are not always of so amiable a character, however,—witness the "Buffonia," a low, noxious weed, growing in marshy places, and named by Linnæus after Buffon, whom he bitterly hated. Indeed, there is a world of meaning hidden under our zoölogical and botanical nomenclature, known only to those who are intimately acquainted with the annals of scientific life in its social as well as its professional aspect.
CTENOPHORÆ.
The Ctenophoræ differ from other Jelly-fishes in their mode of locomotion. All the Discophorous Medusæ, as well as Hydroids, move by a rhythmical rise and fall of the disk, contracting and expanding with alternations so regular, that it reminds one of the action of the lungs, and seems at first sight to be a kind of respiration in which water takes the place of air. The Greeks recognized this peculiar character in their name, for they called them Sea-lungs. Indeed, locomotion, respiration, and circulation are so intimately connected in all these lower animals, that whatever promotes one of these functions affects the other also, and though the immediate result of the contraction and expansion of the disk seems to be to impel them through the water, yet it is also connected with the introduction of water into the body, which there becomes assimilated with the food in the process of digestion, and is circulated throughout all its parts by means of ramifying tubes. In the Ctenophoræ there is no such regular expansion and contraction of the disk; they are at once distinguished from the Discophoræ by the presence of external locomotive appendages of a very peculiar character. They move by the rapid flapping of countless little oars or paddles, arranged in vertical rows along the surface of the disk, acting independently of each other; one row, or even one paddle, moving singly, or all of them together, at the will of the animal; thus enabling it to accelerate or slacken its movements, to dart through the water rapidly, or to diminish its speed by partly furling its little sails, or, spreading them slightly, to poise itself with a faint, quivering movement that reminds one of the pause of the humming-bird in the air,—something that is neither positive motion, nor actual rest.[3]