CHAPTER XI 



HYDRA AND OBELIA. POLYPS AND MEDUSAE. 

 MERISM AND INDIVIDUALITY 



If a handful of weeds gathered from a freshwater pond 

 be placed in a beaker of water and allowed 

 Hydra: t stand for a while, there will often be found 



Features. 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 cleaves 

 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 A touch will cause these to be withdrawn 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, interference with 

 the thread from which they hang is followed by a similar 

 change, and in this way the whole can be made to contract 

 into a vase-shaped mass surmounted by a circlet of little 

 knobs. From time to time water-fleas and other small 

 animals swim against the fine threads and may be seen 

 either to drop through the water as though they were 

 stunned, but afterwards to recover and swim away, or else 

 to remain sticking to the fine threads, which shorten and 

 draw the animal towards the end of the main thread, into 

 which they are swallowed. 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 

 they have been named H. viridis, H. fusca, and H. grisea. 

 The three kinds differ slightly in other respects besides 

 colour, but the following account applies to all of them. 

 The body of Hydra is a hollow cylinder, with a ring of 

 hollow outgrowths or tentacles surrounding an 

 opening or mouth at one end, and the other 

 end closed by a flat basal disc or foot. The mouth is raised 



