OBSERVATIONS ON HYDRA 615 



have remained alive inside the Hydra after the Daphnia 

 had been eaten. 



In damp chambers and on sterile slides the green cells 

 remained alive for a week or more and some divided, but 

 eventually they all died off or bacteria appeared. The cells 

 divided either into two or three. Radais states that in a 

 culture of Chlorella vulgaris the cells divided into four 

 when healthy, but as the culture grew older the rate of division 

 slowed down, and the cells divided into three or two. 



Wlien stained, the organism from H. viridis shows a large 

 and distinct pyrenoid. The nucleus is less distinct and usually 

 appears as an irregular ring of darkly staining material. No 

 division stages were seen. 



Taxonomy. 



The number of species of Hydra has been much discussed 

 ever since the foundation of the genus. Schulze (9) has 

 lately divided it into three genera and about ten species. 

 The Hydras on which I worked do not exactly correspond to any 

 of Schulze's species but come nearest to his H. attenuata, 

 from which they differ in being hermaphrodite. It seems 

 to me improbable that the genus Hydra is justifiably 

 divided up into so many definite species. Some of Hchulze's 

 species are founded on the examination of preserved specimens 

 only. The general habit, colour, size, and so on, are used as 

 differentiating characters, while the nematocysts are always 

 treated as important diagnostically. The Hydras on which 

 I worked varied considerably in size and habit, but all possessed 

 the same kind of nematocysts. In some the egg was stuck on 

 the side of the glass and in some it fell freely to the bottom. 

 Considering the great variation in appearance which may take 

 place within the lifetime of one individual, it seems unsafe to 

 separate off as distinct species animals whose whole life history 

 has not been completely followed through. 



This work was done during my tenure of a Carnegie scholar- 

 ship from 1920-2, in the Natural History Department of 



