THE FRESH-AYATER HYDRA. 169 



paralyzed by means of these threads which are sud- 

 denly shot out; the tentacles then carry the victim 

 to the mouth and it is swallowed. 



Note the simplicity of the structure of hydra — the 

 absence of any distinct nervous system, and all 

 special organs of circulation and respiration. 



Hydras have been cut into slices, lengthwise and 

 crosswise, and each part not only continued to live 

 but grew into a perfect hydra. Hydras have also 

 been turned inside out and in a short time digested 

 food as usual, what had been the outer layer of the 

 body now becoming the lining of a stomach. The 

 tentacles when cut off do not live. 



Besides multiplying by budding, hydra also pro- 

 duces ova and spermatozoa in projections of the body 

 walls. Both kinds of sexual elements are produced 

 in the same individual. Such an animal is called a 

 hermaphrodite. 



There is a large group of animals, almost without 

 exception marine, constructed on essentially the same 

 plan as hydra, though often much more complicated. 

 Hence the hydra is the type of the group known as 

 the Hydroids. Many of them live in colonies, as 

 if the young hydras, instead of dropping off from 

 the parent and becoming distinct individuals, re- 

 mained attached with a free communication between 

 them all. At least two distinct forms of individuals 

 are commonly found : — 



a. A hydra-like form, devoted to obtaining and pre- 

 paring nourishment for the colony, hence called 

 the nutritive zooid. 



b. Modified forms, producing the generative ele- 

 ments, the generative zooids. 



