298 FEESH-WATEE AQUAEIA. 



and shape to a piece of horsehair of about pn. long. One end 

 of this body has a sucking-disc, by means of which the animal 

 is able to affix itself to any foreign substance ; at the other 

 end there is a mouth, which is surrounded by very fine tentacles, 

 varying in number from three or foui- to twelve. These tentacles 

 are capable of great extension, so much so that in some species 

 they may be stretched several times longer than the length of 

 the creatm-e's body. The mouth, which is very elastic, has the 

 food brought to it by the tentacles. Dr. Hogg says that "the 

 organ of prehension, which is called the hasta, consists of a 

 sac opening at the surface of the tentacle, within which, at 

 the lower portion, is placed a saucer-shaped vesicle supporting 

 a minute ovate body, which again bears a sharp calcareous 

 piece called the ' sagitta ' arrow. This can be pushed out at 

 the pleasure of the animal, serving to roughen the surface of 

 the tentacle, and afford a much firmer hold of the living prey.'' 



The tubercles, which more or less cover the tentacles, possess 

 extremely fine threads, called " urticating threads,'' by means 

 of which the Hydra is able to sting its victim to death. These 

 "urticating threads" are so deadly in their action that none 

 of the smaller aquatic animals, upon which this polype feeds, 

 ever seem to recover from the consequences of coming in contact 

 with them. It is a most curious sight to watch a Hydra fishing, 

 as it were, for its food. Its tentacles are extended and swaying 

 to and fro, and directly a water-flea, larva of a gnat, little red- 

 worm, or any such creature comes within touch of one or more 

 of them, the victim seems to be paralysed, for after a few faint 

 struggles it either dies, or resigns itself to its fate as the long 

 and pliant tentacles slowly entwine themselves around the poor 

 creature, and gradually draw it towards the mouth which they 

 surround. Sometimes a Hydra catches at the same time more 

 victims than it feels disposed to eat. It will, in such a case, 

 devour sufficient to satisfy its appetite and relax its hold upon 

 the rest : for the latter, however, there is no escape, as the 

 "urticating threads" will have done their work, and the unfor- 

 tunate animals just released from captivity sink down to the 

 bottom of the water to die — ^that is, if they were not already dead 

 before the polype allowed them, to become disentangled. A red- 



