24 



INTRODUCTION TO ZOOLOGY 



CHAP. 



little rounded mass, with mere knobs to represent the 

 tentacles (Fig. 9, E\ 



Different The colour of the body varies in the different 

 Species, species ; Hydra viridis is bright green, H. fusca a 



brownish colour, and H. vul- 

 gar is a lighter yellowish- 

 brown. The two last species 

 are also larger, and have 

 usually six relatively long 

 tentacles, reaching in H. fusca 

 to several times the length 

 of the body, whilst in H. 

 viridis the eight tentacles, 

 even when fully extended, 

 are not as a rule as long 

 as the body. 



Though usu- 

 ally fixed to one 

 spot, a Hydra can move from 

 place to place, gliding along 

 on the disc at the base of 

 its body, or moving with an 

 action rather like that of a 

 looper caterpillar, throwing 

 its body forward and hold- 

 FIG. 9.-Hydra viridis attached to a stick, ing firmly to some support 

 A, A fully extended individual which is budding. With its tentacles, then de- 



B, A contracted individual. taching the basal end and 



drawing it up ' close to the 



mouth, and fixing it once more (see Fig. 10). In this way 

 the creature can move fairly rapidly from place to place. 



Movement. 



stick. ',, 



FIG. 10. Hydra viridis, showing successive positions when moving 

 from A to B. 



Food. 



The Hydra feeds on minute water animals and 

 plants which, when touched by the waving tentacles, 



