18 



Chapter II 



HYDKA. 



Hydra is a small freshwater animal, abundant in ponds and 

 ditches and in slowly moving streams. 



It consists of a tubular body, about a quarter of an inch 

 in length, one end of which is closed, while the opposite end 

 is open and forms the mouth, a short distance below which is 

 a circle of tentacles, usually six to eight in number. 



Both the tentacles and the body are extremely contrac- 

 tile. The former, when the animal is disturbed, can be 

 almost completely withdrawn, and the latter may shrink up 

 so as to become a mere knob. 



Hydra is usually attached by its closed basal end or foot 

 to water- weeds or other bodies. By contractions of the foot 

 it can crawl along slowly, and it can also progress more 

 rapidly by fixing itself alternately by the mouth and foot, 

 arching the body with a looping movement like a caterpillar. 

 It is carnivorous, and by means of the nematocysts with 

 which its tentacles are studded can paralyse and kill active 

 animals nearly as big as itself and of active habits. 



Hydra receives its name from its remarkable power of 

 recovery from injury. A specimen may be cut into two or 

 more pieces, either transversely or longitudinally, and each 

 fragment will not only survive, but within a short time will 

 become a complete Hydra. The entire animal may even be 

 regenerated from a single tentacle. 



At least three species of Hydra are described as occurring 

 commonly in this country. Of these H. viridis is distin- 

 guished by its green colour and its smaller size, H. fusca is 

 brown in colour, and H. vulgaris almost colourless. 



