AND THE SURFACE-P^ILM OF WATER. 311 



must be water-repellent, a fact which is in harmony with what is 

 known concerning the general surface of the shell in this and 

 many other forms of Cladocera. 



It must not be supposed that S. mucronata always lives sus- 

 pended from the surface-film. On the contrary, the animal spends 

 most of its time, perhaps, in clinging, probably by means of the 

 hooks and setae on its feet, to water plants, etc., and in swimming 

 from place to place. But when the surface of the water in which 

 it lives is calm, and especially when the sun is shining, S. mucronata 

 comes up and remains for long periods in contact with the surface. 

 That the animal obtains some of its food from the miscellaneous 

 organic particles which collect on the surface is rendered almost 

 certain by the fact that if an isolated individual, placed, say in a 

 watch-glass, be observed under a low power of the microscope when 

 at the surface, it can be seen that the particles on the surface are 

 drawn between the valves by the normal beating of the feet. A 

 very minute quantity of flour dusted on the water will show this 

 action more plainly. 



There isl one other point to which attention may be directed, 

 and that is the peculiar dark coloration of the ventral parts of 

 the animal. This is very well brought out in the illustration, and 

 it will be noticed that the coloration not only occurs on the shell, 

 but also on the ventral parts of the large antennae and the post- 

 abdomen. The explanation that suggests itself at once is that 

 this is a case of protective coloration ; for the animal, in con- 

 sequence of this darkening of the ventral surfaces, is certainly 

 much less noticeable when seen from above over a muddy bottom 

 than it would otherwise be. It is not known, however, whether 

 *S'. mucronata has any enemies which prey upon it from above 

 the surface of the water. It may very well be that some of the 

 aquatic insects which spend part of their time on the surface are 

 not unwilling to add this little Entomostracan to their bill of fare, 

 but I have not been able to obtain any direct evidence of this. 



From the foregoing remarks it will be clear that the utilisation 

 of the curious physical properties of the surface-film of water by 

 >S'. viucronaUt is no mere accident, but a very perfect adaptation of 

 means to ends. This is all the more remarkable because the 

 majority of the Cladocera and Ostracoda not only derive no 

 advantage from the surface-film, but find it a positively dangerous 

 factor in their environment, as may be seen by the helpless way 



