THE FRESH-WATER POLYP. 695 



surrounding medium, and an inner layer lining the tube- like interior or 

 stomach cavity. Separating these cell layers there is a very thin intermediate 

 or supporting (non-cellular) layer. This supporting layer by itself might be 

 thought of as a kind of framework exactly corresponding with the shape of the 

 body, for it penetrates even to the tips of the tentacles, which arise as 

 outpushings of .the body wall, and are therefore themselves tube-like. In 

 this intermediate layer are found all the processes of the cellular layers 

 which function as nerves and muscles. At the mouth, the outer and inner 

 cell layers pass into one another ; this is an important point, because, as we 

 shall presently see, the arrangement of the layers at the mouth forms one of 

 the chief distinctions between the Hydrozoa and the Scyphozoa. 



The cells of the outer layer of the Hydra body are chiefly remarkable for 

 their stinging apparatus. The stinging cells contain minute vesicles (Fig. 5), 

 many of them measuring not more than T -^^ of an inch, in which are coiled 

 up stinging threads, which are very valuable weapons of attack and defence. 

 Each of these stinging cells is provided with a minute projection, the cnidocil, 

 and it appears that, when this is touched, the vesicle shoots out its long, 

 coiled, barbed thread so rapidly that, delicate as it is, it is able to penetrate 

 the body of any creature rash enough to approach too near. The poisonous 

 effect of these weapons is shown by the fact that any small animal thus 

 attacked appears to be paralysed; the victim speedily ceases struggling to 

 escape, and is drawn in by the tentacles to the mouth. It is said that each 

 tentacle of a Hydra is provided with many thousands of such weapons, 

 which are being constantly produced afresh, new cells taking the place of 

 those which have been discharged. 



The cells constituting the inner lining of the body are provided with fine 

 hairs or whips, and are capable of changing their shape, and of sending out 

 processes into the cavity of the body ; they are therefore called amoeboid 

 cells. The simple crustacean or other prey which, caught and paralysed by 

 the stinging tentacles, has been pushed through the mouth into the cavity of 

 the body, is digested by the cells of this lining layer. Whatever cannot be 

 assimilated is thrown out again at the mouth, the muscles of the body wall 

 and the current of water kept up by the whips in the cavity assisting in this 

 process. 



The Hydra often reproduces itself by means of buds which grow out from 

 the parent body, and when fully equipped for independent life, break off and 

 attach themselves near the parent. But, at certain seasons, a large globular 

 body forms at some part of the surface of the fully-grown Hydra. This 

 represents an egg, and, when fertilised, separates from the parent and 

 develops into a new Hydra. 



The Hydra and its allies have a wonderful power of repairing injuries 

 which the body may suffer. Almost any part of the body, if cut off, is able 

 to reproduce the rest. Some naturalists have amused themselves by produc- 

 ing monstrous growths possessed of a large number of heads, and others have 

 tried the experiment of turning the sac-like body inside out, and have even 

 asserted that the Hydra is able to carry on its life activities in such a condi- 

 tion. It has, however, been proved that, under such circumstances, the 

 Hydra either manages to regain its normal state or else dies. 



In this account of the Hydra, which is also known as the Fresh- water polyp, 

 in contradistinction to other marine polyps, such as the Sea-anemones, the 

 Corals, and the Zoophytes, we have noticed some of the essential points in 

 the structure of the Cnidarian body. Among these are the sac-like nature 



