HYDROIDS. 



3 



that he is still quoted in the latest works as authority. The hydra is 

 found generally in fresh water, though some few species have been 

 discovered, in this country, in that which is somewhat brackish. It 

 loves still or slowly-running water, and attaches itself generally to 

 the under-side of the leaves or to the stalks of aquatic plants. Its 

 body is extremely contractile, and consists, like the oceanic hydroids, 



Fig. 12. Hydra vulgaris. (Natural Size.) 



in its earliest stage of development, of a simple elongated sac, with 

 an opening which answers the purpose of a mouth. Around the 

 mouth are a series of hollow filaments which it can entirely withdraw, 

 and it then looks like a minute tubercle. The tentacles are roughened 

 by the clusters of thread-cells, or cnida, already described. The 

 threads have been observed in some instances to be, when extended, 

 as much as eight inches long, and are shot out, it is thought, by the 

 propulsive power of a liquid injected into the central cavity. It 

 grows erect, horizontal, or inverted, as the case may be, and lives 

 only upon animal food. The little creatures are extremely voracious 

 and not over-nice. Trembley observed two hydras attack, at the 



