There was no bread upon the table.
"My wisdom is confirmed," said Aaron; and he gave me the delectable substitute, Sophie's handiwork.
METHODS OF STUDY IN NATURAL HISTORY.
XIV.
If I succeeded in explaining my subject clearly in the last article, my readers will have seen that the five Orders of the Echinoderms are but five expressions of the same idea; and I will now endeavor to show that the same identity of structural conception prevails also throughout the two other Classes of Radiates, and further, that not only the Orders within each Class, but the three Classes themselves, Echinoderms, Acalephs, and Polyps, bear the strictest comparison, founded upon close structural analysis, and are based upon one organic formula.
We will first compare the three Orders of Acalephs,—Hydroids being the lowest, Discophorae; next, and the Ctenophorae highest. The fact that these animals have no popular names shows how little they are known. It is true that we hear some of them spoken of as Jelly-Fishes; but this name is usually applied to the larger Discophore, when it is thrown upon the beach and lies a shapeless mass of gelatinous substance on the sand, or is seen floating on the surface of the water. The name gives no idea of the animal as it exists in full life and activity. When we speak of a Bird or an Insect, the mere name calls up at once a characteristic image of the thing; but the name of Jelly-Fish, or Sun-Fish, or Sea-Blubber, as the larger Acalephs are also called, suggests to most persons a vague idea of a fish with a gelatinous body,—or, if they have lived near the sea-shore, they associate it only with the unsightly masses of jelly-like substance sometimes strewn in thousands along the beaches after a storm. To very few does this term recall either the large Discophore, with its purple disk and its long streamers floating perhaps twenty or thirty feet behind it as it swims,—or the Ctenophore, with its more delicate, transparent structure, and almost invisible fringes in parallel rows upon the body, which decompose the rays of light as the creature moves through the water, so that hues of ruby-red and emerald-green, blue, purple, yellow, all the colors of the rainbow, ripple constantly over its surface when it is in motion,—or the Hydroid, with its little shrub-like communities living in tide-pools, establishing themselves on rocks, shells, or sea-weeds, and giving birth not only to animals attached to submarine bodies, like themselves, but also to free Medusae or Jelly-Fishes that in their turn give birth again to eggs which return to the parent-form, and thus, by alternate generations, maintain two distinct patterns of animal life within one cycle of growth.
Perhaps, of all the three Classes of Radiates, Acalephs are the least known. The general interest in Corals has called attention to the Polyps, and the accessible haunts of the Sea-Urchins and Star-Fishes have made the Echinoderms almost as familiar to the ordinary observer as the common sea-shells, while the Acalephs are usually to be found at a greater distance from the shore, and are not easily kept in confinement. It is true that the Hydroids live along the shore, and may be reared in tanks without difficulty; but they are small, and would be often taken for sea-weeds by those ignorant of their true structure.
Thus this group of animals, with all their beauty of form, color, and movement, and peculiarly interesting from their singular modes of growth, remains comparatively unknown except to the professional naturalist. It may, therefore, be not uninteresting or useless to my readers, if I give some account of the appearance and habits of these animals, keeping in view, at the same time, my ultimate object, namely, to show that they are all founded on the same structural elements and have the same ideal significance. I will begin with some account of the Hydroids, including the story of the alternate generations, by which they give birth to Medusae, while the Medusae, in their turn, reproduce the Hydroids, from which they spring. But first, a few words upon the growth of Radiates in general.
There is no more interesting series of transformations than that of the development of Radiates. They are all born as little transparent globular bodies, covered with vibratile cilia, swimming about in this condition for a longer or shorter time; then, tapering somewhat at one end and broadening at the other, they become attached by the narrower extremity, while at the opposite one a depression takes place, deepening in the centre till it becomes an aperture, and extending its margin to form the tentacles. All Radiates pass through this Polyp-like condition at some period of their lives, either before or after they are hatched from the eggs. In some it forms a marked period of their existence, while in others it passes very rapidly and is undergone within the egg; but, at whatever time and under whatever conditions it occurs, it forms a necessary part of their development, and shows that all these animals have one and the same pattern of growth. This difference in the relative importance and duration of certain phases of growth is by no means peculiar to the Radiates, but occurs in all divisions of the Animal Kingdom. There are many Insects that pass through their metamorphoses within the egg, appearing as complete Insects at the moment of their birth; but the series of changes is nevertheless analogous to that of the Butterfly, whose existence as Worm, Chrysalis, and Winged Insect is so well known to all. Take the Grasshopper, for instance: with the exception of the wings, it is born in its mature form; but it has had its Worm-like stage within the egg as much as the Butterfly that we knew a few months ago as a Caterpillar. In the same way certain of the higher Radiates undergo all their transformations, from the Polyp phase of growth to that of Acaleph or Echinoderm, after birth; while others pass rapidly through the lower phases of their existence within the egg, and are born in their final condition, when all their intermediate changes have been completed. We have appropriate names for all the aspects of life in the Insect: we call it Larva in its first or Worm-like period, Chrysalis in its second or Crustacean-like phase of life, and Imago in its third and last condition as Winged Insect. But the metamorphoses of the Radiates are too little known to be characterized by popular names; and when they were first traced, the relation between their different phases of existence was not understood, so that the same animal in different stages of growth has frequently been described as two or more distinct animals. This has led to a confusion in our nomenclature much to be regretted; for, however inappropriate it may be, a name once accepted and passed into general use is not easily changed.
That early stage of growth, common to all Radiates, in which they resemble the Polyps, has been called the Hydra state, in consequence of their resemblance to the fresh-water Hydra to be found in quantities on the under side of Duck-Weed and Lily-pads. For any one that cares to examine these animals, it may be well to mention that they are easily found and thrive well in confinement. Dip a pitcher into any pool of fresh water where Duck-Weed or Lilies are growing in the summer, and you are sure to bring up hundreds of these fresh-water Hydrae, swarming in myriads in all our ponds. In a glass bowl their motions are easily watched; and a great deal may be learned of their habits and mode of life, with little trouble. Such an animal soon completes its growth: for the stage which I have spoken of as transient for the higher Radiates is permanent for these; and when the little sphere moving about by means of its vibratile cilia has elongated a little, attached itself by the lower end to some surface, while the inversion of the upper end has formed the mouth and digestive cavity, and the expansion of its margin has made the tentacles, the very simple story of the fresh-water Hydra is told. But the last page in the development of these lower Radiates is but the opening chapter in that of the higher ones, and I will give some account of their transformations as they have been observed in the Acalephs.