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