2 ME. D. J. SCOTIBFIELD ON ENTOMOSTEACA 



elusion o£ special details and the tentative nature of some of the 

 ideas advanced, it is hoped that the present paper will prove, on 

 the whole, of some general interest. 



Neglecting the Phyllopods, which have not been specially 

 studied, and the Copepods, which will be referred to later, the 

 remaining Entomostraca — Cladocera and Ostracoda — present so 

 much in common from the point of view of relation to the 

 surface-film, that it will be convenient to consider them together. 

 Por instance, to a large number of animals belonging to both 

 these Orders the surface-film is an ever-present source of danger. 

 It must have been noticed by all collectors of pond-life that 

 whenever a gathering is made containing an abundance of these 

 forms, in a very short time a number of them will be found 

 floating on their sides at the surface in a helpless condition, 

 apparently quite incapable of getting back into the water by 

 their own exertions. From observations made on isolated speci- 

 mens it appears that the main chance such animals have of 

 regaining their normal habitat lies in moulting. If this be 

 possible to the animals shortly after their misadventure, they can 

 slip back into the water, leaving their cast carapaces still floating 

 at the surface. But if they are not nearly ready to moult, and 

 are also unable to get back by other means, such as violent dis- 

 turbance of the surface by the wind, for example, it need hardly 

 be pointed out that such an unnatural position can only mean a 

 more or less speedy death. The chief sufierers in this respect, 

 so far as my observations go, seem to be species pf the genera 

 Daphnia, Ceriodaphnia, Simocephalus, Sosmina, Eurycercus, and 

 Acroperus among the Cladocera, and of Cypria, Cypris, and Ser- 

 petocypris among the Ostracoda, although others also may some- 

 times be found in this unfortunate state. Judging from the 

 usual paucity of these floating forms on the surface of open 

 waters, it seems probable, however, that, under natural conditions, 

 only a comparatively few lives are sacrificed in this way. But on 

 this point more extended inquiry is necessary before a definite 

 statement can be made. That an enormous number of "Water- 

 fleas may occasionally perish from this cause, is clearly shown by 

 the following instance : — During the last summer Baphnia 

 Schcefferi, Baird, occurred in astonishing abundance in the 

 London Docks, and when I visited the latter in July there was 

 a dark red scum, composed entirely of these animals, forming a 

 border from 1 to 2 feet in width along the quays, around the 



