AND THE SUEFACE-FILM OF WATER. d 



ships, &c., and even forming patches of many square yards in the 

 narrow channels connecting the " basins." 



"Whatever its ultimate value may be found to be as an element 

 in the life-histories of the various species, the means by which 

 this helpless floating at the surface is brought about is well worth 

 examination. That it cannot depend upon any single circumstance 

 seems probable from the following considerations : — (1) The 

 animals, although small, have a decidedly greater specific gravity 

 than water, as can be seen by the way in which they sink upon the 

 stopping of their swimming-organs when not in contact with the 

 surface ; (2) they are completely immersed before the floating 

 takes place ; and (3) they possess considerable muscular energy, 

 which, if it were not counteracted by other circumstances, would 

 probably take them quickly below the surface. It will be found, 

 I believe, that there are several factors contributing in varying 

 degrees to the observed result. In the first place, the animals 

 subject to this undesirable connection with the surface-film have 

 highly-polished water-repellent shells. This can be directly seen 

 by an examination of floating specimens, and can be further 

 verified by experiment. !Por instance, if a fair-sized Daphnia or 

 Eurycercus or Cypris, that has been floating, be placed upon a 

 glass slip with a small drop of water, it will be found that a narrow 

 pointed strip of blotting-paper may be applied to the upperside 

 of the animal without getting wet. Under similar conditions, 

 bodies not possessing water-repellent surfaces would have a film 

 of water passing completely over them, and the blotting-paper 

 would therefore draw up a continuous stream. Secondly, it is a 

 well-known fact that when a substance is partly immersed in a 

 liquid which cannot wet it, the surface-film of the liquid is drawn 

 downwards at the line of contact to form a descending capillary 

 curve. The effect of this, as explained in most text-books of 

 physics, is that the surface-film exerts an upward pull upon the 

 body against which the descending capillary curve is formed. 

 By putting these two points together, the first and most im- 

 portant step in the explanation of the floating of these Ento- 

 mostraca may be made. On the one hand, there are the animals 

 with water-repellent shells, and, on the other, the property of 

 the surface-film to form a capillary depression when in contact 

 with water-repellent substances. The process, therefore, must 

 be as follows : — "When an Entomostracan having one of these 

 waterproof jackets happens to pierce the surface-film, a capillary 



1* 



