720 



THE NATURE BOOK 



forms a loop, brings its head and arms in 

 contact with whatever it may be resting 

 on. The Hydra, grasping this support 

 with its arms, now relaxes the hold which 



At certain seasons of the year, a glass 

 of water drawn from a pond will be 

 found to contain a large number of 

 more or less oval-bodied creatures, which 



attached the base of the body, and slowly move about very rapidly in the water with 



turns a complete somersault. In this way, 



alternately using its arms and " foot," 



the little creature moves along Iiy a series 



of stately somersaults until its destination 



is reached. 



With the aid of a good hand magnify- 



a curious jerkj^ motion. From their 

 quaint mode of progression, these little 

 creatures have gained their popular name 

 of Water Flea, although really they are 

 not insects at all, but distant relations of 

 the shrimp and lobster, belonging to 



ing glass, we can make out more clearly that group of animals which are included 



some of the characteristics of tlie Hydra, in the Crustacea. Occasionally they 



In the centre of the waving circlet of arms appear in such vast quantities as to give 



is an opening, which is the mouth of the the water quite a milky appearance, and 



creature, and which leads straight down finding their way into the domestic water 



to the digestive 

 chamber. Close in- 

 spection of the arms, 

 or tentacles, will re- 

 veal the fact that 

 they are more or 

 less thickly covered 

 with very complex 

 cells, the stinging 

 cells, or nematocysts, 

 by means of which 

 the Hydra wounds 

 and captures its 

 prey. These stinging 

 cells are similar to 

 those found on the 

 tentacles of the Sea 

 Anemone, which I 

 have already de- 

 scribed on page 380. 

 If a healthy and 

 well-nourished Hydra is kept under ob- 

 servation in a small glass jar, a little 

 nodule will be seen to make its appear- 

 ance on the side of the animal's body, 

 and to grow outwards and develop a 

 crown of tentacles which wave about 

 and capture any minute organisms that 

 come within their reach. This 

 Hydra grows, and after a time, becoming 

 detached from the parent, settles down 

 on its own account. Sometimes the Hydra 

 develops a waist, which becomes smaller 

 and smaller until the top half breaks off. 

 This upper, tentacle-crowned portion then 

 develops a base, while the fixed lower por- 

 tion grows a crown of tentacles. In the 

 autumn the Hydra produces eggs, from 

 which escape the young, which gradually 

 develop their tentacles. 



FRESH-WATER HYDRA. 



pipes, cause con- 

 siderable trouble. 



The movements 

 and general appear- 

 ance of the Water 

 Flea, or Daphnia, to 

 give it its scientific 

 name, are very in- 

 teresting to watch 

 with the aid of a 

 good hand magnify- 

 ing glass, or, better 

 still, under the micro- 

 scope. It will then 

 be seen that the little 

 animal is enclosed in 

 a transparent, oval or 

 nearly circular shell, 

 terminating at the 

 base in a sharp 

 serrated spine. So 

 transparent is the shelly casing of the body, 

 that most of the internal anatomy of 

 the Water Flea can be easily distinguished, 

 and under the microscope, with a moderate 

 magnification, the heart may be seen 

 beating. The eye of the Water Flea is 

 a very bright and handsome organ, and 

 baby looks like a tiny jewel. 



The Water Flea belongs to the division 

 of the Crustacea called the ErJomoslraca, 

 and this division includes, as well as the 

 Water Flea, other small and interesting 

 inhabitants of the pond, called respectively 

 the Cypris and the Cyclops. The little 

 Cypris differs considerably in size and 

 a]i])carance from the Water Flea ; the shell 

 is not quite so oval, but rather more like 

 the seed of a haricot bean, or a kidney, in 

 shape ; and is generally fringed or covered 



