XXI MOVEMENTS 221 



Besides these movements of contraction and expansion, 

 Hydra is able to move slowly from place to place. This it 

 usually does after the manner of a looping caterpillar (Fig. 

 49, c) : the body is bent round until the distal end touches 

 the surface : then the base is detached and moved nearer the 

 distal end, which is again moved forward, and so on. It has 

 also been observed to crawl like a cuttle fish (d) 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 from about a millimetre downwards in length, 

 belonging to the class Crustacea, a group which includes 

 lobsters, crabs, shrimps, &c. 



Water-fleas swim very rapidly, and occasionally one may be 

 seen to come in contact with a Hydra's tentacle. Instantly 

 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 tentacle 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 



