58 PERIOD III. 



rest, a small green stalk, barely visible to the naked 

 eye, was found attached to one of the plants. From 

 one end of the stalk filaments or tentacles were seen to 

 project, and these moved slowly about. When the 

 vessel was shaken the stalk and tentacles contracted, 

 but soon extended themselves again. Was this object 

 a plant or an animal? Its shape and colour were those 

 of a plant, and sensitive plants were known which 

 drooped when touched or shaken. Further observation 

 showed that it could move from place to place, W 7 hich 

 favoured the animal interpretation. . Trembley deter- 

 mined to cut the stalk in two ; if the halves lived when 

 separated the fact would favour the plant-theory. The 

 halves at first gave no signs of life beyond occasional 

 contraction and expansion, but after eight days small 

 prominences were seen on the cut end of the basal 

 half. Next day the prominences had lengthened ; on 

 the eleventh day they seemed to be growing into 

 tentacles. Before long eight fully formed tentacles 

 were visible, and Trembley had two complete specimens 

 in place of one ; both were able to move about. 



After four years of observation a handsome quarto 

 volume was published, which told the history of " The 

 freshwater Polyp," a name suggested by Reaumur ; 

 the Latin name of Hydra was given by Linnaeus. 

 Hydra had been discovered and slightly described forty 

 years before by Leeuwenhoek, who had seen two young 

 polyps branching from one parent and spontaneously 

 becoming free. Trembley made out all that a simple 

 lens, guided by a skilful hand and a keen eye, could 

 discover. Thirteen plates were admirably engraved by 

 another amateur, Pierre Lyonet, who was in all respects 

 a fit companion for Trembley. It was proved that 

 Hydra preyed upon living animals, especially upon the 



