80 



ILLINOIS ACADEMY OF SCIENCE 



Animals living beneath stones show a preference for weak 

 light, those living on stones, medium light, those among stones, 

 strong light. If we were to study the community in full we 

 would find that reactions to many other factors are of impor- 

 tance.. Associative memory no doubt plays a role. Thus there 



SAND-GRAVfl BOTTOM OR POOL COMMUNITY 



WHOLLY OR PARTIALLY 

 BURIED IN SUBSTRATUM 



POSITIVE ,WEAKLIGHT| 



REACTIONS TQ< 



'/////A MEDIUM LIGHT H 8URR0WING 



SAND \^.X : \ CURRENT ^^ 



Figure 9 

 Biol. Bull. Vol. XXVI, Fig. 4, p. 314. 



is agreement in reaction to factors of prime importance in the 

 community habitat as a whole and disagreement in respect to 

 factors differing strikingly in the different situations in which 

 the animals live within the community habitat. 



The diagram of the pool community, Fig. 9, is introduced to 

 show how strikingly it differs from that of the rapids com- 

 munity. Though agreement is not indicated here on account 

 of the incomplete character of the experiments, our experi- 

 ence with the reactions of pool fishes and invertebrates to 

 chemical differences in water, suggests that such differences 

 may be of much importance to all the species. The difference 

 between the two communities is emphasized by the presence of 

 a strong preference for sand bottom and by the presence of the 

 burrowing habit, both of which are wanting among the animals 

 of the rapids community. The non-burrowing pool species 

 are positive to current ; the burrowing species do not respond 

 within ordinary lengths of time. The reactions to light show 

 much more sharp negativeness than in the case of darters and 

 crayfishes of the pool community. The community is clearly 

 unsolved and a large amount of experimentation would be 

 necessary to determine suitable tests for these animals and then 

 all the animals of both communities should be put through all 

 the tests, new and old. A series of new tests must be added 

 for each new aquatic community and all the old tests must be 



