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THE AMERICAN NATURALIST [Vol. XLIX 



Physa five times and Planorbis four times. These snails 

 varied in size from two to twenty-five mm. long. Except 

 in one instance, they were in shaded pools or came out on 

 cloudy days. The exceptional case may be considered as 

 similar because the luxuriant growth of watercress near 

 the algae furnished spots of shade, although most of the 

 "blanket" was in sunlight. It seems fair to assume that 

 snails are not regular inhabitants of the surface algae, but 

 merely forage there when there is little or no sunlight. 



Many small crustaceans were observed. Chydorus and 

 Bosmina were numerous, while two other Cladocera, Daph- 

 nia and Simocephalus, were less in evidence The ostra- 

 cods found were in eleven collections and quite numerous. 

 They have not been identified. Cyclops, Canthocamptus 

 and Diaptomus were the copepods identified. Cyclops 

 was remarkably constant and abundant. Many females 

 bearing paired egg-sacs and many copepod nauplii, pre- 

 sumably young cyclops, were among the number. The 

 adults were from one to three mm. long. The Isopod, 

 Asellus aquaticus, was found only once and then in a mass 

 of algae close to a mud bank. Two amphipods, Gam- 

 marus and Hyalella, were observed several times. 



The last group of foraging animals and the one to which 

 the largest individuals of this population belong is the 

 Insecta. In this class were found larvae, nymphs and 

 adults, representing five orders of insects. Three nymphs 

 of Callibcetis, in pool A, one of Bcetis, in pool M, and ten 

 of undetermined genera of Ephemerida in Pool F were 

 the only may-fly nymphs found. The Odonata were more 

 frequent. There were a few Libellulids, and a number of 

 nymphs of Enallagma and Ischnura, The Hemiptera had 

 only one representative, Corisa, the water-boatman, which 

 was caught twice but was frequently seen swimming on the 

 clean surface of the pool. It can hardly be considered a 

 regular inhabitant of the alga-masses. 



Four different larvae of the order Diptera made up the 

 greater part of the insect population. Chironomus was 

 particularly conspicuous, since the larvae were found con- 



