310 D. J. SCOURFIELD ON SCAPHOLEBERIS MUCEONATA 



the surface of the water, to which a Scapholeheris is cHnging, be 

 looked at in such a way that the light is totally reflected towards 

 the eye. They then appear as four minute irregularities produced 

 by the anterior and posterior groups of setse on each valve. It is 

 true that these irregularities cannot be actually seen to be 

 depressions, but they certainly cannot be elevations, because it is 

 a well-known physical fact that only when capillary depressions 

 are formed can a weight be supported by the surface-film. But 

 that is exactly what is happening in the case of a Scwpholeheris 



attached to the surface of the water, for on the one hand its body 

 is decidedly heavier than water, and on the other it can be readily 

 observed that the animal does not use its swimming antennae when 

 in contact with the smface, except for a stroke now and again to 

 alter its position laternlly. 



It is quite certain, therefore, that the projecting seta? of the 

 ventral margins of the valves in Scapholeheris do pierce the surface- 

 film and produce capillary depressions, which, although very 

 minute, are sufficiently large to support the difference in weight 

 between the animal's body and the water which it displaces. It 

 also follows that the sette which produce these capillary depressions 



