358 THE POPULAR SCIENCE MONTHLY 



and form, may be observed. To these cells the appropriate name of 

 " thread-cells," or cnidce, has been given. When their structure is 

 investigated, each little cell is seen to .possess an elastic wall of dou- 

 ble nature; the inner layer of the wall being strong, while the outer 

 one is of thinner and more delicate texture. The upper or open ex- 

 tremity 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 con- 

 tained fluid escapes, and the thread and its sheath are quickly pro- 

 truded 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 appropri- 

 ate 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 zoophytes, 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 inca- 

 pable 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 proba- 

 bility 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 Coelenterata for thus the HydrcB^ 

 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 Acalephce, or "nettle-like" 

 animals. And bathers and swimmers, through instinct, if not thor- 

 ough zoological knowledge, generally and wisely contrive to give the 

 jelly-fishes a wide berth in their marine meanderings. The late Ed- 

 ward Forbes, in his huraorsome manner, says of one species of jelly- 

 fish, that, " once tangled in its trailing ' hair,' the unfortunate, who 



