OBSERVATIONS ON HYDRA 607 



The size of the nematocysts has been cited as a charac- 

 teristic difference for the various species. Most authors, 

 however, give relative sizes only, and all measurements are 

 given for the penetrants alone, which, as will be seen, are 

 much the most variable in size of all the types. Steche (10) 

 states that the penetrants of H. fusca are at most 8-8-5 /n, 

 and those of H. grisea at least lO-Sjix and usually 13-13-5ju,. 

 Toppe (12) says that those of H. fusca are the smallest, 

 H. grisea next, and those of H. attenuata the largest. 

 Schulze (9) gives 25 /x for his H. attenuata (which is not 

 the same species as Toppe's) and 13/x for Pelmatohydra 

 oligactis. 



In my Hydra the size of the penetrant varies greatly. 

 Some forms measure lOju, or 11/x and others may be as large 

 as 22 /i. Even in one individual the sizes may vary by as much 

 as Sjx. The small penetrants are found in the tentacles as well 

 as in the body and are not merely incompletely developed. 

 It is difficult to speak with certainty, but the most frequent 

 size seems to be about 15ju,; 12-1 3 ju, and 18-1 9 ju, are com- 

 moner than the intermediate sizes. In small Hydras the 

 smaller size seems to be more frequent than the large. 



The glutinants vary much less, the ranges of individual 

 variation not being more than 3 or A fi. The larger type 

 (streptoline) measures about 11-5/x (maximum 13 and minimum 

 10/i), and the smaller (stereoline) about 9/x (7-11 ju,). 



The volvents measure from 5-10 jit. The usual size is about 

 8)u and the smaller sizes are more numerous than the larger. 



In order to see w^hether the size of the nematocysts varied 

 from time to time with changes in the size of the individual, 

 I took a large well-grown Hydra, and after removing two of 

 the tentacles in order to measure the nematocysts, starved it 

 for seventy-three days. The measurement of the width of the 

 foot when the animal was fully extended was, before starvation, 

 about 0-275 mm., and, after, about 0-100 mm. It had therefore 

 decreased to almost a third of its original width, and this gives 

 a rough indication of the general effect. Whereas, before 

 starvation, the nematocysts were of normal size (penetrants 



