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NATURE STUDY REVIEW [9 :4— April, 1913 



studying the material. This may be done as follows : Fasten a 

 thistle tube to the apex of a small miller's bolting cloth net, cut 

 off the thistle tube close to the bowl and place a rubber tube 

 over the free end. Close this with a pinch cock. Pour the 

 water containing the entomostracans into this net and carefully 

 wash all down into the bowl of the thistle tube. In this way 

 a great many animals may be secured in a small amount of water. 

 These animals may be collected from any Chicago city water 

 tap by allowing the water to run over night into one of these mil- 

 ler's bolting cloth nets. 



Cladocera, copepods, and ostracods all exhibit a daily vertical 

 migration such as has been noted for most of the other groups. 



As before, light is probably the major 

 cause of this migration. However, it 

 has been shown that for some at least 

 of the copepods this reaction has a 

 different explanation. The females 

 were found to be positive to gravity 

 in the light and negative in darkness 

 and the males simply follow the 

 females but have no other reaction 

 that would cause them to give this 

 migration if isolated. All three of 

 these groups are positive to light of 

 a medium intensity and will collect 

 just at the edge of a shadow. Al- 

 though they are normally positive to 

 Fig. 13.— Cladoceran. Bos- light they are easily reversed in their 

 mina. (After Herrick.) reaction to it. Thus when daphnia 



are placed in a dish lighted from 

 one side at first they will all swim to the lighted side, but after 

 being exposed a time some become negative to light and sooner 

 or later all become negative. Thus it is possible to remove all the 

 daphnia from such a dish by picking out the negative ones with a 

 pipette. Shaking some ostracods or even picking them up in a 

 pipette and dropping them out has been shown to reverse their 

 reaction to light. 



Economic Importance. 



These crustaceans are of importance economically. All are 

 food for fishes and in fact they make up the major part of the 

 food of most of our important food fishes. Although all are eaten 

 by fish, yet the distinction of being the real basis of fish food 

 supply is held by the small entomostracans. Not only are these 

 eaten directly by the fishes, but they are the basis of almost every 



