HYDRA AND OBELIA 



HYDRA 



If a handful of weeds gathered from a freshwater pond be placed 

 in a beaker of water and allowed to stand for a while, there will 

 often be found hanging from the sides of the beaker or from the 

 weeds some short threads of a green, brown, or whitish colour. 

 By one end each thread 

 sticks to the glass. At the 

 other it bears about half a 

 dozen finer threads, which 

 hang down in the water if 

 they be left undisturbed. A 

 touch will cause these to be 

 withdrawn and take on a 

 shorter and thicker shape, and 

 a stronger blow will cause 

 the whole structure to con- 

 tract into a subspherical knob, 

 surmounted by a crown of 

 smaller knobs. It is clear that 

 these objects are living beings : 

 in point of fact each of them 

 is a specimen of the animal 

 known as Hydra. According 

 to their colour and other 

 characters they have been 

 divided into a number of 

 species and subgenera. Chloro- 

 hydra viridissima {=H . viridis) 

 is green. Hydra oligactis [=H. 



fusca) is brown and has a stalk, and tentacles up to 25 cm. long ; 

 and H. attenuata, also brown, has no stalk, and the tentacles 

 are only twice the length of the body. The following account 

 applies in general to all of them. 



Fig 



6. — Two specimens of Chlorohydra 

 viridissima x c. 6, one contracted, the 

 other in a state of moderate expansion, 

 the latter bearing two buds in different 

 stages. 



»»., Mouth ; or.c, oral cone. 



M.Z. 4 



89 



