RHEOTAXIS IN ISOPODA 295 



quiet pools or other places of lodgement which keep the isopods 

 from being swept down stream in the breeding season. The nor- 

 mal rheotactic reactions of animals from the two types of habitats 

 are quite different. Animals from streams give a high percentage 

 of positive responses; they are much more vigorous and definite 

 in their reactions and show a long period of positive reaction 

 in a straight current. The pond isopods on the other hand give 

 a weak positive response and are much inclined to be indifferent 

 to the current. They are less active than the stream mores, and 

 in the straight current, they give only a short positive response. 



The most obvious difference in the environment in the two 

 habitats is the difference in the oxygen content of the water. In 

 the streams this is normally between 5 and 10 cc. per liter depend- 

 ing on temperature, rate of flow, and the character of the stream 

 bed. In the ponds studied, on the other hand, the amount of 

 oxygen present is low, seldom going above 3 cc. per liter at any 

 period of the year. There is also a difference in the amount of 

 free carbon dioxide present in the two habitats. This runs about 

 2 cc. per liter in the streams while in the ponds it has been taken 

 as high as 40 cc. per liter, the usual amount however is about 

 10 to 15 cc. per liter. From this work alone it would seem prob- 

 able, that the difference in positive response to the current in 

 the two habitats may be due to the difference in the oxygen or 

 carbon dioxide content of the water. 



The main breeding season occurs in the spring of the year in 

 both habitats. In the pond isopods, this does not affect the 

 normal rheotactic response so markedly as with the Aselli from 

 the stream. In these it causes a distinct lessening of the positive 

 reactions so that for the time being, they behave as though they 

 were pond isopods. The young from both habitats give a low 

 positive response, but in the streams the reaction becomes more 

 positive as the animals increase in size, while in the pond isopods it 

 remains the same throughout life. • 



