Page:Popular Science Monthly Volume 51.djvu/535

Rh matter stained with these pigments. Other species have been noticed, but they are less common, and some are rarely met with.

A single individual may be thus described: There is the body, like a thick thread, of varying length up to an inch. At the foot the substance is slightly expanded into a small disk, with which the creature fixes itself to some surface; at the top is a series of arms or tentacles, thin, threadlike, and arranged symmetrically around the end of the stem, which is the mouth. These tentacles vary in number, the green hydra having from six to ten, and the brown one from seven to twelve. As the trunk sways this way and that in a slight degree, and the tentacles twist about, the sight is a curious one and well worth while watching. The arms have the power of contracting, and sometimes look like little buds around the mouth; and the trunk itself will also sink down until it seems to be a small mass of jelly.

The trunk is nothing but a stomach. There are no lungs, no liver, no heart, no intestines, no nervous system. All there is consists of two layers of tissue, with a very delicate layer between. The trunk and the tentacles are alike in structure, and are simple hollow tubes. If the latter are examined with a magnifying glass, they are found to be covered with little warts, which are technically called "urticating" or stinging organs. These consist of an oval capsule, the top of which is turned back into itself so as to form a cavity in which there lies a thread coiled up, while round the edge of the cavity are four little darts. In this introverted capsule the thread lies bathed in a poisonous secretion, and the darts are nothing more or less than poisoned arrows. The opening is provided with a sort of trigger, so that when any substance is pressed against it the capsule is flung outward, the thread is cast round the opposing body, and the poisoned arrows are projected into its substance if it is penetrable.

The hydra lives on minute aquatic creatures, and is exclusively carnivorous. Attached to its moorings, it spreads its arms in every direction with a searching motion; and although the two commonest forms can not explore at a much greater distance than the height of the body, the "fuscous" species—one of the rare forms—has tentacles which can reach some seven or eight inches. As soon as a victim touches the subtle angling line it is seized by it, enveloped in the threads, and struck by the