Forevv^ord 



"Further, I discovered a little animal whose body was at times 

 long, at times drawn up short, and to the middle of whose body .... 

 a still lesser animalcule of the same make seemed to be fixed fast 

 by its hinder end .... [At that time the little animalcule] had only 

 four very short little horns, yet after the lapse of sixteen hours I 

 saw that its body and its horns had increased in bigness, and four 

 hours later still I saw it had forsaken its mother.''^ 



In this remarkable letter written on Christmas Day, 1702, An- 

 tony van Leeuwenhoek amazed members of the Royal Society by 

 annonncing a discovery of dual significance. While reporting the 

 initial description of the organism which we now call hydra," he 

 also described the first instance of asexual reproduction ever ob- 

 served in animals. Thus, from their very discovery hydra have 

 served to reveal new biological phenomena. 



More startling findings with hydra followed when, in 1744, 

 Abraham Trembley published in his superb Memoires an exposition 

 of: the first controlled experiments on regeneration; the first suc- 

 cessful animal grafting experiments; the first investigations of photo- 

 taxis in lower invertebrates; the first vital staining of tissues; and 

 thorough proof of asexual reproduction by budding. Two centuries 

 have passed since Trembley made these revolutionary discoveries, 

 an interim during which research on hydra was sporadic, and hydra 

 were relegated to a subsidiary role in classroom instruction. In the 



1 Letter 149, December 25, 1702. Quoted in Antony van Leeuwenhoek and his 

 "Little Animals" by Clifford Dobell, Dover Pubis., N. Y., 1960, pp. 280-281. 



- In this volume we have adopted, whenever possible, the following usages for 

 purposes of unifonnity and clarity: (a) Hydra, when referring to one or more speci- 

 mens of this genus if the species has already been clearly indicated; (b) hydra, 

 when referring to one or more specimens of the Hydridae in general, and when the 

 species is not indicated; (c) hydras, when referring to a number of genera of the 

 Hydridae. 



