THE MICROSCOPE. 



attitudes white, red, light flesh, or beautiful grass- 

 green. The body of the hydra is of a gelatinous 

 nature, altering in shape as it changes its position ; 

 when contracted, in some species a mere tubercle with 

 short radiating papillae ; when extended, becoming a 



narrow cylinder. 

 One end expands 

 and forms an ad- 

 herent disc j the 

 other has a mouth 

 surrounded by nu- 

 merous exceedingly 

 contractile arms or 

 tentacles, varying 

 in number accord- 

 ing to the age and 

 species of the in- 

 dividual. The 

 hydra's body is 

 composed of two 

 membranes, techni- 

 cally termed ecto- 

 derm and endoderm, 

 the former being 

 the external cover- 

 ing, the latter the 

 internal lining of the cavity. The tentacles are tubes, 

 which are, in fact, prolongations of these two mem- 

 branes. They are the arms by which the animal seizes 

 its prey, and they are placed a little, below the mouth, 

 which, when closed, protrudes like a snout above them. 

 Both membranes have irregularly rounded nodules on 

 their surface. These nodules, especially in the tenta- 

 cles, contain capsular bodies (thread-cells), in which 

 may be seen (the hydra being crushed between bits of 

 glass, under a high microscopic power) curious organs. 



Hydra attached to a Weed. 



