34 Instinct and Intelligence 



Hydromedusae. This class of animals is of 

 particular interest, as it includes the lowest 

 order of beings possessing nerve cells, together 

 with definite sensory organs, or structures 

 specially adapted to receive, and transmit to 

 nerve cells various modes of energy acting on 

 the surface of the animal's body, the result of 

 which frequently becomes manifest in instinc- 

 tive movements. 



The Hydromedusae present two main forms, 

 the non-sexual polyps or Hydra, and the sexual 

 Medusae, such as jelly-fish and sea anemones; 

 as an example of the former we may refer to 

 the small yellow or light brown fresh-water 

 polyps known as Hydra fusca. The body of 

 this creature consists of a cylindrical sac, its 

 walls being formed by an outer and inner layer 

 of cells, with an intervening gelatinous con- 

 tractile substance. This Hydra is commonly 

 found attached by one end of its body or foot 

 to some solid substance, such as a water plant ; 

 the other end of the body opens to the exterior 

 by an orifice which leads to the central cavity 

 or digestive department of the animal. Round 

 the Hydra's mouth from six to nine tapering 



