16 ME. D. J. SCOITEEIELD ON ENTOMOSTEACA 



This assumption is perhaps less necessary in the case of Noto- 

 dromas because of the general prevalence of a nearly straight 

 ventral margin among the Ostracoda, but it is essential for 

 ScapJioleberis. Thirdly, of freshwater forms having nearly 

 straight ventral margins, only those Cladocera accustomed to 

 swim in a somewhat reversed position, and those Ostracoda 

 swimming in the same way, or at least vertically, could, it would 

 seem, have taken advantage of the surface-film by means of their 

 ventral shell-margins. 



There is only one other point that need be mentioned before 

 leaving this j)art of the subject. It has been pointed out in a 

 previous part of the paper that to many of the Cladocera the 

 surface-film is a source of danger ; yet to these very same forms 

 there is one indirect way in which the surface-film is probably 

 beneficial, not perhaps to individuals, but to the species. The 

 envelopes of the ephippial or resting eggs of these creatures 

 possess the same water-repellent power characteristic of the 

 carapaces from which they are developed, and in virtue of this 

 they are often found floating on the surface, although really of 

 greater density than water. By this means their dispersal must 

 be greatly facilitated, and their transmission from pond to pond 

 rendered possible, even without the drying-up of the particular 

 pieces of water in which they are produced. 



The relation of the Copepoda to the surface-film of water 

 is all that now remains to be considered. The first fact that 

 presents itself in this connection is that never by any chance can 

 an animal belonging to tbe freshwater division of this order be 

 found floating on the surface in the helpless condition common 

 among the Cladocera and Ostracoda. This may seem puzzling 

 at first, but the apparent explanation is that the coverings of 

 these Copepods do not repel water. A similar experiment to 

 that already mentioned, where a little strip of blotting-paper 

 was applied to the body of an animal lying in a minute quantity 

 of water on a glass-slip, will prove that the water extends in a 

 film quite over the body of such a form as Cyclops or Diaftomus. 

 "Why it is that with presumably the same covering-material the 

 bodies of some Entomostraca should thus exhibit no power of 

 water-repulsion, while the bodies of others, as already shown, are 

 highly water-repellent, remains quite unknown. 



In spite of the foregoing peculiarity of their coats, some 

 Copepods are able to suspend themselves from the surface-film. 



