52 



FRESH-WATER BIOLOGY 



Numbers of animals select the same environment because of 

 physiological similarity. All the animals occupying a relatively 

 uniform habitat constitute an animal community. A physiological 

 agreement exists among the animals of a community. The rapids 

 community of a large creek is in a general agreement in reactions 

 to certain factors, and disagreement in respect to factors differ- 

 ing in intensity vertically. In Fig. 5 is shown a noteworthy agree- 

 ment in reaction to bottom and current under experimental condi- 



^oSTRATA 



OPEJy WATER 

 AMONG 

 STONES 



Son stones 



V UNDER 

 ^STONES 



{STRONG current! 

 HARD BOTTOM | 

 MEDIUM LIGHT 



UNDER STONES R^ 

 ON STONES ESISI 

 STRONG LIGHT ^^ 



AMONG STONES f?^ 

 WEAK LIGHT ^^ 

 KINAESTHESIA tHJ 



Fig. 5- 

 To show the agreement and disagreement of the reactions of the animals of the rapids community. 

 Note agreement of reaction to bottom and current and disagreement in two other reactions related to the 

 level at which the animals live. These results were obtained by placing the animals under experimental 

 conditions in which they had a choice between different kinds of bottom, different strengths of light, and 

 in which their behavior in a water current was noted. In the case of water current the percentage of ani- 

 mals headed upstream is given. When headed upstream animals are said to be positive to current. In 

 the case of the other stimuli the percentage of animals in the kind of conditions available was noted and the 

 animals are said to be ixjsitive to the conditions in which tlie greatest number are found. Thus note 

 that the darter (ElheoUoma) was 80 per cent among the stones and is said to be positive to this kind of 

 situation. It will be noted that if the animals had been 100 per cent positive to the various stimuli the 

 entire 400 units would be occupied in the diagram. This could be true only if there were no other factors 

 entering into the reactions ol the animals. The common names of the animals are as follows: Etheo- 

 iiotna, darter, Cambarus, crayfish; Goniobasis, snail; Ilydropsyche, caddice worm; Argia, damsel tly; 

 Perla, stone lly; neptagemna, may-lly sub-family; Pscphenus, water penny. 



tions. The preference for hard bottom in these experiments means 

 the avoidance of sand as only sand and hard bottom were present 

 in the experiments. Animals living under stones were under stones 

 m darkness in the experiments. The snail (Goniobasis) which lives 

 on stones was found on stones in the experiments. The darter 

 (Etheostoma) and the crayfish (Cambarus) which live among stones 

 were found among stones in the experiments. Thus the different 



