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Note on Scapholeberis mucronata and the Surface-film 

 OF Water. 



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



The habits of the different species of Entomostraca are very 

 much more varied than is commonly supposed. Some forms swim 

 continuously in the open water of clear ponds and lakes, some 

 attach themselves in various ways to weeds, some crawl about the 

 bottom, some burrow in the mud, some live habitually in wet 

 mosses, and so on. But of all the peculiar modes of existence, 

 that of deliberately making use of the ceiling of a pond, i.e. the 

 surface-film of water, for support, is probably the most remarkable. 

 So far as is known, only'a very few species have acquired this 

 power in a fully developed fashion, and these are all included in 

 two genera — namely, Scapholeberis (Cladocera) and Notodromas 

 (Ostracoda). 



The accompanying figure, which is reduced from a diagram used in 

 illustration of an exhibit made at the Club's Conversazione held at 

 Queen's Hall on May 4th, 1897, shows a specimen ( X 70 diameters) 

 of Scapholeberis mucronatct {Daphnia mucronata of Baird) in the act 

 of clinging to the surface-film. The mechanism by which this 

 is accomplished seems to be as follows. On the perfectly straight 

 and flattened ventral margin of each valve there exists a series of 

 very peculiarly modified setae, the anterior and posterior members 

 of which are larger and project somewhat more than the rest. 

 (Full details of these characteristic set?© will be found in my 

 paper on "Entomostraca and the Surface-film of Water," pub- 

 lished in the Journal of the Linnean Society, Zoology, vol. xxv., 

 1894, pp. 1-19.) When the animal, which habitually swims in 

 a reversed position, brings its ventral margin into contact with 

 the surface of the water, the setse which project farthest from 

 the shell pierce the surface-film and produce minute capillary 

 depressions. These depressions can be seen with a pocket lens if 



