176 THE BOTTOM OF THE SEA. 



7. Hydra, type of the Hydrozoa or Hydra Polyps— Extraordinary 

 properties of the Hydra discovered by Trembley— Marine 

 Hydrozoa. 



The hydrozoa owe their name to a special type, 

 the hydra, which inhabits fresh waters. The cele- 

 brated naturalist Trembley, tutor to the sons of 

 Count Bentinck in Holland, was the first to re- 

 cognise their nature, in 1740. The remarkable pro- 

 perties which he discovered among these little beings 

 struck the learned of the eighteenth century with 

 astonishment. The flesh-eating habits and the spon- 

 taneous movements which he had remarked among 

 the hydrae led him to believe they were animals, while 

 their appearance resembled that of aquatic plants. 



Trembley made the following decisive experi- 

 ment. Plants have the property of reproducing 

 themselves by cuttings — that is to say, a branch of 

 the plant being cut off, and planted under suitable 

 conditions, roots develope themselves at the cut ex- 

 tremity, and the branch becomes a plant similar to 

 its parent. As no known animals possessed thL«> 

 singular faculty, it was to be presumed that the 

 hydra would not reproduce itself by cuttings. What 

 then was the astonishment of Trembley when he 

 observed, some days alter the mutilation of a polyp, 

 each morsel transformed into a complete body. 



