and sufficiently buoyant to keep the animal on a level with the surface of the water; and the vertical plate,

rising into the air, acts like a sail, by which the creature is driven rapidly along. From the lower surface of the body hang down numerous long, dark-blue, tentacular appendages, or cirrhi, disposed in several rows, by the motion of which the animal can change its direction, or move along when there is not wind enough to catch its tiny sail. It is a fearless navigator, boldly venturing,

"Like little wanton boys, that swim on bladders,"

across the widest and deepest ocean. Perhaps none of the Jelly-fishes have a more extended geographical range. Their centre appears to be in the warmer parts of the ocean, and they are sent northwards and southwards into high latitudes of either hemisphere by the force of the great oceanic currents. On the west coast of Ireland, especially toward the close of the summer, vast numbers of Velella are driven on shore, entangled

192

REPRODUCTION OF PLANTS AND ANIMALS.

in floating sea-weed, and very frequently accompanied by the beautiful Ianthina, or "Blue Snail-shell," a singular mollusc that equally "swims on bladders." In the Velella of our shores the sail is immoveable, and the vessel is therefore much at the mercy of the winds, but there is an exotic species which is said to have the power, by the contraction of muscular bands, to lower its sail at pleasure.

Many circumstances in the history of the ACALEPHÆ are calculated to excite curiosity or admiration, but, perhaps, there is no fact connected with them more wonderful than the mode of their reproduction from gemmæ or buds. It is a character of the vegetable kingdom, that its organisms propagate themselves in two ways: one by seeds, formed by special organs called flowers; the other by gemmæ or buds, which may be developed from any part of the cellular substance of the plant. Both modes of reproduction effect a similar object, the continuance of the species: -but it is observable that individual characters are more strictly perpetuated when plants are multiplied by buds than when they are grown from seeds; hence, one mode of growth is said to be a multiplication of the individual plant, the other, a propagation of the species through new individuals. In the higher classes of animals propagation by gemmæ does not occur: the young are brought forth either in a fully-formed state, or in eggs, from which they will in time be hatched. As we descend lower in the series we find the process considerably varied, and become familiarized with certain transformations through which the young creature passes

before it acquires its full complement of limbs. Even before we take leave of the Vertebrates, there are extraordinary examples of such transformations. Thus in the race of Frogs: the young, or tadpole, is deficient in limbs, swims like a fish, and breathes through gills; while the full-grown animal is, as every one knows, furnished with nimble and well-formed legs, and breathes through lungs. One can scarcely conceive a greater change in organization than is here displayed before our eyes. It strikes us as wonderful, because the young of other Vertebrates exhibit no such change after birth; and yet it would appear, from the researches of anatomists, that before birth the foetus of all, not excepting that of man himself, undergoes changes of an analogous nature. So that here, as everywhere, nature vindicates her uniformity. All of the vertebrate class are destined to go through a certain round of changes, but in some a portion of these changes take place before birth, in others after it.

Leaving the Vertebrates, in which transformation of the young after birth is the exception, we reach the Articulate or Insect races, in which it becomes the rule. All are familiar with the quadruple state under which insect-life appears, the egg, the grub or caterpillar, the chrysalis, and the perfectly-formed insect. In these, as in the frog, we find the young animal fitted for a condition of life totally different from that to which its mature state is destined; and, in many cases, the difference in its breathing apparatus is equally great. The young of many insects, as of the dragon-flies and gnats, live under water until their last change, when, rising to

the surface, they cast aside their skin, with its gills and fins, and thenceforward breathe the air through which they fly. Similar changes we have already noticed in the Crustacea, and such we may have to speak of in other classes of animals, but these are not of the same nature as what we have now to describe as taking place in the ACALEPHE, or Jelly-fishes. The insect deposits an egg, and each egg will, in due time, produce an insect similar to its parent, and nothing more. But the Jelly-fish throws off organized bodies, which can scarcely be called eggs, but which may more justly be compared to the gemmæ or buds of a plant; for, from every one of them may spring a whole colony of Jelly-fishes. The extraordinary history of these creatures was first ascertained by M. Sars, a celebrated Swedish naturalist. The English reader may find a more detailed account than is here given in Steenstrup's "Alternation of Generations," published by the Ray Society, and in a very interesting memoir by Dr. Reid in Taylor's "Annals.” *

Without adopting all the theoretical inferences deduced from the "alternation of generations," we may state the facts as follows. The Medusa gives birth to a multitude of minute gelatinous bodies, in shape not very unlike the so-called eggs of a sponge, or the spores of one of the lower Alge and, like them, furnished with a multitude of cilia, or vibratile hairs, which clothe the surface, and by their motion propel the little body through the water. These active little bodies must, I think, be looked upon as gemmæ or buds, rather

"An. Nat. Hist." (1848), p. 25, &c. See also Forbes" "Monograph of the British Naked Eyed Medusa, Ray Society," 1848.

MEDUSE FROM BUDS.

195

than as young Medusa, properly so called. When emitted, the bud is of an oval shape, broader at one end; and it constantly keeps its broader end in advance when moving. Internally they present a cavity. They are at this stage bags of living jelly, clothed with vibratile hairs. After a while the bud attaches itself by its larger extremity, or apparent front, to any convenient object,— as a stone or the stalk of one of the larger sea-weeds,— and this extremity henceforward becomes the base on which all its future operations are conducted. When it has become fixed by this base an alteration of form quickly commences. The body lengthens, and becomes wider upwards; and, at its upper extremity, is formed a mouth, which at first, is of small size and naked, but gradually becomes larger and surrounded by four prominences.

Y000

days new tentacula

make their appearance between the old ones,

MEDUSE-BUDS IN VARIOUS STAGES.

and these organs, developed successively, one set after the other, are gradually increased to the number of twenty-eight or thirty. We have now the appearance of an animal resembling one of the more simple polypes, such as the Hydra,-a bell-shaped, gelatinous, bag-like body, fixed to a stalk, highly contractile in every part, and furnished with a mouth surrounded by tentacula.