and form, may be observed. To these cells the appropriate name of "thread-cells," or cnidœ, has been given. When their structure is investigated, each little cell is seen to possess an elastic wall of double nature; the inner layer of the wall being strong, while the outer one is of thinner and more delicate texture. The upper or open extremity of the inner layer of the sac is prolonged to form a kind of sheath, which protects and gives origin to a thread-like filament, from the presence of which, indeed, these cells derive their name. This thread, in the ordinary condition of the cell, is coiled up within the interior of the sac, and around its own sheath; and in many cases both thread and sheath may be discerned to be provided with minute spines or hooks. The cell itself, in addition, contains a fluid, amid which the thread is submerged.
Such is the essential structure of a thread-cell in its normal state of what we may term repose. When such a structure, however, is pressed or irritated in any way, the cell ruptures or bursts, the contained fluid escapes, and the thread and its sheath are quickly protruded or thrown out from the opening in the cell. If, now, the thread and fluid are observed to come in contact with any body of appropriate and assailable kind, such a body will exhibit certain symptoms which will indicate to us the probable nature of these curious cells. Thus, when the tentacles or feelers of the sea-anemone, or of any of the zoöphytes, come in contact with a minute or susceptible organism adapted for food, the organism is first observed to struggle to escape from the entwining filaments which encircle its body. Soon, however, its active exertions cease, and the victim appears paralyzed and incapable of helping itself, or of struggling longer with its captor. The thread-cells, in other words, have been discharging their miniature darts or "threads" into the body attacked; the fluid—in all probability of acrid or poisonous nature—has been poisoning the tissues of the struggling organism; and the observation has revealed to us that the functions of the cells are undoubtedly analogous to those of the serpent's fangs and poison-gland, in that they serve to paralyze and kill the prey.
As might naturally be supposed, the power of the thread-cells varies in different species and groups of the animals that possess them; but there are some forms of Cœlenterata—for thus the Hydrœ, sea-anemones, and their allies, are collectively named—in which the stinging-cells are of sufficient size and power to inflict severe pain on man himself. Aristotle was fully aware of this latter fact when he named the jelly-fishes and their allies Acalephœ, or "nettle-like" animals. And bathers and swimmers, through instinct, if not thorough zoölogical knowledge, generally and wisely contrive to give the jelly-fishes a wide berth in their marine meanderings. The late Edward Forbes, in his humorsome manner, says of one species of jellyfish, that, "once tangled in its trailing 'hair,' the unfortunate, who