Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/34

28 upside down, and swims off, stomach hanging downward from an umbrella or bell-shaped body; we would then have a jelly-fish.



Every one who has visited the sea-shore must have had his attention called to the jelly-fishes (Fig. 20), as they floated along by means of the pulsations of their disc-bearing bodies, the animal looking somewhat like an umbrella, and he remembers well the sensations suffered while bathing when his skin came in contact with the long streamers floating about, which are so characteristic of these organisms. The stinging is due to a poison which is contained in vesicles situated in the skin, often millions in number. In the beautiful Blue Physalia, known to sailors as the Portuguese man-of-war, this poison is so powerful that it has been known to cause death. The jelly-fishes and Anemones alike possess these poison-cells; but the Sponges, although having the rudimentary canal system, are devoid of the stinging structures. The most simple example of the Hydrozoa is our common Green Hydra (Fig. 18), so called from the fact that when cut in pieces each piece becomes a new individual. It looks to the naked eye like a piece of green silk thread. When magnified, it is seen to be a simple tube, the digestive cavity and general cavity of the tube being the same; its mouth is surrounded by a circle of arms or tentacles, by means of which it seizes its prey, paralyzing or destroying it by the poison just spoken of The importance of the Hydra, as part of the evidence for the evolution of life, maybe seen in the development of the so-called Hydroid jelly-fishes, such as the Campanularia and Sertularia (Fig. 19), which look like little trees covered with flowers. The branches of these tree-like beings are little tubes, in which the flowers (Hydroid polyps) live; the tubes are all connected, so the colony has a common