TREMBLEY 281 



the ovum, but the proof was not complete, as other 

 polyps had been kept in the same vessel. Koesel a few 

 years later figured the ovary and the ripe ovum within 

 it, but was not fortunate enough to see polyps emerge 

 from the ova ; he thought, quite truly, that the young 

 polyps which are plentiful in spring originate in this 

 way, but was too cautious to affirm what he could not 

 prove. ^ 



Live specimens were sent to Reaumur, who had no 

 hesitation in placing them among animals, and calling 

 them polyps, the old Greek name for cuttle-fishes. 

 Linnaeus afterwards proposed the generic name oi Hydra, 

 suggested by the fabulous monster which sprouted out 

 new heads, as fast as the old ones were cut off. 



In his memoir Trembley describes three species, the 

 green, the common brown, and the long-armed brown 

 Hydra. He says that, besides preying upon Daphnia, 

 his Hydras devoured Nais, Cypris, insect-larvae and 

 pupae, and even very small fishes. Fishes and also the 

 whirligig-beetle rejected the Hydra after once biting it ; 

 the irritating thread-cells were of course unknown as 

 yet. Trembley mentions the power which the polyp 

 possesses of fixing its body by means of its tentacles, 

 and shows that it can travel looping- fashion, attaching 

 the fore and hind ends alternately ; he was not aware 

 that it can also move slowly by means of the foot alone. 

 He describes the suspension from the surface-film, and 

 shows that a single tentacle can thus hold up the body. 

 A drop of water let fall upon a Hydra floating in this 

 way sends it to the bottom at once. There is also 

 mention made of the infusorian parasite (Trichodina), 

 which runs about on the Hydra. 



^ Insectm-Bdustigungen, Theil III, pp. 500, 513-4, pi. 83, figs. 1, 2; 



89, fig. 7. 



