HYDRA. 471 



length when seeking for prey; and on coining in contact 

 with any object floating through the water, they imme- 

 diately twine round it, and convey it to the mouth. In 

 some genera the tentacles appear to be tubular, the inter- 

 nal cavity being continuous with that of the stomach. 

 The mouth is situated in the centre of the circle of ten- 

 tacles, and leads directly into a simple digestive cavity. 



Hydras are found in ponds and rivulets, adhering to the 

 leaves of aquatic plants, or twigs and sticks that have 

 fallen into the water. When stretched out, they resemble 

 pieces of hair, from a quarter to three-quarters of an inch 

 in length. Some are of a light-green colour, and others 

 brown or yellow; that is, the five varieties found in 

 England. It received its name from its several long arms 

 being supposed to resemble the fifty-headed water-serpent 

 called Hydra, which was destroyed by Hercules in the 

 lake of Lerna, as we are informed in fabulous history. 

 Leeuwenhoek, in 1703, first drew attention to the Hydra; 

 and in 1739, M. Trembley from the Hague, more accu- 

 rately described its habits. 



Polypes are not vegetarians ; M. Trembley fed Hydrce 

 on minced fish, beef, mutton, and veal; they are voracious 

 and active in seizing worms and larvae much larger than 

 themselves, which they devour with avidity. They care- 

 fully and adroitly bring their food towards their mouth; 

 and when near, pounce upon it with eagerness. To make 

 up for the want of teeth, the mouth enlarges to receive 

 the food brought to it by the arms that have twined around 

 the sacrifice. The red worm that tinges the mud of the 

 Thames appears to be the dainty dish they like best to 

 have set before them. Dr. Mantell saw the lasso of a 

 polype thrown over two worms at the same time; yet 

 they could not escape, and lost all power of motion. 



Dr. Johnston states : " Sometimes it happens that two 

 polypes will seize upon the same worm, when a struggle for 

 the prey ensues, in which the strongest gains, of course, 

 the victory; or each polype begins quietly to swallow his 

 portion, and continues to gulp down his half, until the 

 mouths of the pair near, and come at length into actual 

 contact. The rest that now ensues, appears to prove that 

 they are sensible of their untoward position, from which 



