137 



Hydra and the Surface-Film of Water. 



By D. J. Scourfield, F.R.M.S. 



{Read Oct. IStJi, 1901.) 



Plate 8 (Upper Portion). 



The fact that the different species of Hi/cha, although decidedly 

 heavier than water, can maintain themselves for lengthened 

 periods suspended from the surface film is well known to all 

 observers of minute aquatic life, and various suppositions as to 

 the means by which this peculiar phenomenon is brought about 

 have been advanced from time to time. I do not think, however, 

 that any of the explanations hitherto given have been satis- 

 factory, although I believe, and shall try to show in the 

 following remarks, that the action is very simple, being due, 

 in fact, to the formation of minute capillary depressions in 

 the surface-film by means of a water-repellent substance on the 

 basal disc of the animal. 



In investigating this subject the first thing to do is evidently 

 to examine under the microscope the appearance which the 

 animal presents when clinging to the surface-film. In passing it 

 may not be superfluous to mention that if an animal cannot be 

 induced of its own accord to attach itself to the surface-film in a 

 vessel suitable for observation, such as a glass trough or a flat 

 bottle, it is usually easy to make it do so, for all that is necessary 

 is to take it out of the water, say with a hooked needle, and then 

 gently lower it again into the water. It will almost invariably 

 be found that by such procedure the Hydra will remain suspended, 

 and the observer will not be long in incidentally noticing that 

 it is practically always the basal disc which is attached to the 

 surface and rarely any other part of the body. 



