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.
[Illustration: Coryne mirabilis, natural size]
On shells and stones, on sea-weeds or on floating logs, there may often be observed a growth of exquisitely delicate branches, looking at first sight more like a small bunch of moss than anything else. But gather such a mossy tuft and place it in a glass bowl filled with sea-water, and you will presently find that it is full of life and activity. Every branch of this miniature shrub terminates in a little club-shaped head, upon which are scattered a number of tentacles. They are in constant motion, extending and contracting their tentacles, some of the heads stretched upwards, others bent downwards, all seeming very busy and active. Each tentacle has a globular tip filled with a multitude of cells, the so-called lasso-cells, each one of which conceals a coiled-up thread. These organs serve to seize the prey, shooting out their long threads, thus entangling the victim in a net more delicate than the finest spider's web, and then carrying it to the mouth by the aid of the lower part of the tentacle. The complication of structure in these animals, a whole community of which, numbering from twenty to thirty individuals, is not more than an inch in height, is truly wonderful. In such a community the different animals are hardly larger than a good-sized pin's head; and yet every individual has a digestive cavity and a complete system of circulation. Its body consists of a cavity inclosed in a double wall, continuing along the whole length of each branch till it joins the common stem forming the base of the stock. In this cavity the food becomes softened and liquefied by the water that enters with it through the mouth, and is thus transformed into a circulating fluid which flows from each head to the very base of the community and back again. The inner surface of the digestive cavity is lined with brownish-red granules, which probably aid in the process of digestion; they frequently become loosened, fall into the circulating fluid, and may be seen borne along the stream as it passes up and down. The rosy tint of the little community is due to these reddish granules.