v STRUCTURE 297 



by means of its tentacles, the body being kept nearly 

 vertical. 



It is also possible to watch a Hydra feed. It is a very 

 voracious creature, and to see it catch and devour its prey is 

 a curious and interesting sight. In the water in which it 

 lives are always to be found numbers of "water-fleas," 

 minute animals of about a millimetre or less in length, 

 belonging to the class Crustacea (see Chapter VII). 



Water-fleas swim very rapidly, and occasionally one may 

 be seen to come in contact with a Hydra's tentacle. In- 

 stantly its hitherto active movements stop dead, and it 

 remains adhering in an apparently mysterious manner to the 

 tentacle. If the Hydra is not hungry it usually liberates its 

 prey after a time, and the water-flea may then be seen to 

 drop through the water like a stone for a short distance, but 

 finally to expand its limbs and swim off. If, however, the 

 Hydra has not eaten recently, it gradually contracts the ten- 

 tacles until the prey is brought near the mouth, the other 

 tentacles being also used to aid in the process. The water- 

 flea is thus forced against the apex of the hypostome, the 

 mouth expands widely and seizes it, and it is finally passed 

 down into the digestive cavity. Hydrae can often be seen 

 with their bodies bulged out in one or more places by 

 recently swallowed water-fleas. 



The precise structure of Hydra is best made out by cutting 

 it into a series of extremely thin sections and examining them 

 under a high power of the microscope. The appearance 

 presented by a vertical section through the long axis of the 

 body is shown in Fig. 76, A. 



The whole animal is seen to be built up of cells, each 

 consisting of protoplasm with a large-nucleus (B D, nu\ and 

 with or without vacuoles. As in the case of most animal cells, 

 there is no cell-wall. 



