13 



Chapter n. 

 HYDEA. 



Htdea is a small fresliwater 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 contractile. 

 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 completely withdrawn, and the latter may shrink up so as to 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 or button. 



Hydra is usually attached by its closed basal end or foot 

 to water-weeds or other foreign 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 Hke a cater- 

 pillar. It is carnivorous, and by means of the nematocysts 

 with which its tentacles are studded 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. The entire animal may even be regene- 

 rated 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. 



