v STRUCTURE 297 



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 of about a millimetre or 

 less in length, belonging to the class Crustacea (see 

 P- 386). 



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 tentacles 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 micro- 

 scope. 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, 

 mi), and with or without vacuoles. As in the case of 

 most animal cells, there is no cell- wall. 



