13 



CHAPTER n. 

 HYDEA. 



HYDRA is a small freshwater animal found abundantly 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 and attached to weeds, 

 twigs, etc., 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 contractile. 

 The former, when the animal is disturbed, can be almost 

 completely withdrawn, and the latter shrinks up so as to 

 form a mere knob or button. 



Hydra is usually attached by its closed basal end or foot 

 to some foreign body. 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 car- 

 nivorous, and by means of its tentacles can paralyse and kill 

 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 become 

 a complete Hydra. 



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. 



