GENERAL SKETCH OF ACALEPHS.
In the whole history of metamorphosis, that wonderful chapter in the life of animals, there is nothing more strange or more interesting than the transformations of the Acalephs. First, as little floating planulæ or transparent spheres, covered with fine vibratile cilia, by means of which they move with great rapidity, then as communities fixed to the ground and increasing by budding like the corals, or multiplying by self-division, and later as free-swimming Jelly-fishes, many of them pass through phases which have long baffled the investigations of naturalists, and have only recently been understood in their true connection. Great progress has, however, been made during this century in our knowledge of this class. Thanks to the investigations of Sars, Dujardin, Steenstrup, Van Beneden, and many others, we now have the key to their true relations, and transient phases of growth, long believed to be the adult condition of distinct animals, are recognized as parts in a cycle of development belonging to one and the same life. As the class now stands, it includes three orders, highest among which are the Ctenophoræ, so-called on account of their locomotive organs, consisting of minute flappers arranged in vertical comb-like rows; next to these are the Discophoræ, with their large gelatinous umbrella-like disks, commonly called Jelly-fishes, Sun-fishes, or Sea-blubbers, and below these come the Hydroids, embracing the most minute and most diversified of all these animals.
These orders are distinguished not only by their striking external differences, but by their mode of development also. The Ctenophoræ grow from eggs by a direct continuous process of development, without undergoing any striking metamorphosis; the Discophoræ, with some few exceptions, in which they develop like the Ctenophoræ from eggs, begin life as a Hydra-like animal, the subsequent self-division of which gives rise, by a singular process, presently to be described, to a number of distinct Jelly-fishes; the Hydroids include all those Acalephs which either pass the earlier stages of their existence as little shrub-like communities, or remain in that condition through life. These Hydroid stocks, as they are sometimes called, give rise to buds; these buds are transformed into Jelly-fishes, which in some instances break off when mature and swim away as free animals, while in others they remain permanent members of the Hydroid stock, never assuming a free mode of life. All these buds when mature, whether free or fixed, lay eggs in their turn, from which a fresh stock arises to renew this singular cycle of growth, known among naturalists as "alternate generations."
The Hydroids are not all attached to the ground,—some like the Physalia (Portuguese man-of-war), or the Nanomia, that pretty floating Hydroid of our own waters, move about with as much freedom as if they enjoyed an individual independent existence. As all these orders have their representatives on our coast, to be described hereafter in detail, we need only allude here to their characteristic features. But we must not leave unnoticed one very remarkable Hydroid Acaleph ([Fig. 24]), not found in our waters, and resembling the Polyps so much, that it has long been associated with them. The Millepore is a coral, and was therefore the more easily confounded with the Polyps, so large a proportion of which build coral stocks; but a more minute investigation of its structure (Figs.[ 25], [26]) has recently shown that it belongs with the Acalephs.[2] This discovery is the more important, not only as explaining the true position of this animal in the Animal Kingdom, but as proving also the presence of Acalephs in the earliest periods of creation, since it refers a large number of fossil corals, whose affinities with the millepores are well understood, to that class, instead of to the class of Polyps with which they had hitherto been associated. But for this we should have no positive evidence of the existence of Acalephs in early geological periods, the gelatinous texture of the ordinary Jelly-fishes making their preservation almost impossible. It is not strange that the true nature of this animal should have remained so long unexplained; for it is only by the soft parts of the body, not of course preserved in the fossil condition, that their relations to the Acalephs may be detected; and they are so shy of approach, drawing their tentacles and the upper part of the body into their limestone frame if disturbed, that it is not easy to examine the living animal.
[2] See "Methods of Study," by Prof. Agassiz.
| Fig. 24. Branch of Millepora alcicornis; natural size. (Agassiz.) | Fig. 25. Animals of M. alcicornis expanded; magnified. a a small Hydroid, larger Hydroid, t tentacles, m mouth. (Agassiz.) | Fig. 26. Transverse section of a branch, showing pits, a a a a, of the large Hydroids with the horizontal floors. (Agassiz.) |
The Millepore is very abundant on the Florida reefs. From the solid base of the coral stock arise broad ridges, branching more or less along the edges, the whole surface being covered by innumerable pores, from which the diminutive animals project when expanded. ([Fig. 25.]) The whole mass of the coral is porous, and the cavities occupied by the Hydræ are sunk perpendicularly to the surface within the stock. Seen in a transverse cut these tubular cavities are divided at intervals by horizontal partitions ([Fig. 26]), extending straight across the cavity from wall to wall, and closing it up entirely, the animal occupying only the outer-most open space, and building a new partition behind it as it rises in the process of growth. This structure is totally different from that of the Madrepores, Astræans, Porites, and indeed, from all the polyp corals which, like all Polyps, have the vertical partitions running through the whole length of the body, and more or less open from top to bottom.