AND THE SUKPACE-FILM OP WATEK. 15 



In addition to the several characters in common already given, 

 there is one other which merits a passing notice, depending, as it 

 probably does, largely upon the similar habits of the two species 

 under consideration. It has been observed that in this country 

 both are to be found only during the warmer half of the year *. 

 This is not a very striking fact in the case of Scapholeheris, 

 because such a limitation of the period of activity is the rule 

 rather than otherwise among the Cladocera ; but it is more 

 noticeable with Notodromas, as the Ostracoda do not furnish 

 many other examples of periodicity, and even such occur appa- 

 rently in the colder, rather than in the warmer, part of the year. 

 It will readily be seen that the periodicity found, to exist is very 

 advantageous in both these particular cases, for the power the 

 animals possess of attaching themselves to the surface-film would 

 be nearly useless during most of the winter, owing either to the 

 ice or to the comparatively disturbed state of the surface of the 

 water when not frozen. 



The surface-utilising habit is so very peculiar and so strangely 

 limited among the Cladocera and Ostracoda, that any evidence 

 relative to its origin would possess more thau ordinary interest. 

 Possibly nothing definite will ever be known about the stages of 

 its evolution, but certain suggestions may be made which seem 

 to throw a little light upon the matter, or at any rate to con- 

 siderably narrow the problem. In the first place, it appears 

 almost certain that the habit did not arise under marine con- 

 ditions, because a nearly smooth water-surface is an essential, 

 even now, for its exhibition. Secondly, it is tolerably certain that 

 of freshwater forms only those having approximately straight 

 ventral margins could have been able, in the first instance, to 

 use the surface-film to advantage. This, coupled with the fact 

 that both Scapholeheris and Notodromas can attach themselves to 

 the sides of a glass vessel with their ventral margins towards the 

 glass, leads me to think that the forms from which the present 

 species have been derived were in the habit of crawling over the 

 surfaces of weeds, &c., much as Graptoleberis tesUtdinaria, 

 [Fischer, does now, and had been modified in the same direction. 



* See Baird's • Natural History of the British Entomostraca,' pp. 100 and 

 154. Also the author's paper on " The Entomostraca of Wanstead Park," in 

 the Journal of the Quekett Microscopical Olub, ser. 2, vol. v. p. 165. 



