RHEOTAXIS IN ISOPODA 341 



the isopod, regarding the time distance from the moulting period. 

 The exact bearing of the moulting cycle upon rheotaxis has not 

 yet been worked out, but evidence is accumulating to show that 

 in stream isopods, the moulting period and the time immediately 

 following it are characterized by an indefinite or at any rate a 

 weak rheotactic response. 



Regarding the permanence of the modifications produced, it 

 has been repeatedly mentioned, that in pond isopods kept in 

 high oxygen, there was no reversal to the normal pond response, 

 although in two cases these experiments ran for over six months. 

 On the other hand, the stream isopods seemed to possess a power 

 of acclimatization, and a return to the normal stream positive- 

 ness in a fairly low oxygen content. However, in the case of the 

 isopods reared entirely in low oxygen, this increase in positiveness 

 was much less marked. Apparently if the stock could have been 

 carried through a few more generations the response would have 

 been entirely that of pond isopods. Hence it would appear that 

 the reactions, with which this paper is dealing, are distinctly 

 dependent upon the environment for their continuance. On 

 the other hand the taxonomic differences, as the number of spines 

 on the propodus of the first thoracic appendage, show the same vari- 

 ation in both habitats. That is, the taxonomic differences are 

 inheritable characteristics of the species, and are not dependent 

 upon external conditions, while the behavior characters here 

 studied are almost independent of heredity. 



The size of the isopods has been shown to be correlated with the 

 amount of oxygen in the water and size is often used in defining 

 taxonomic species. So here we have one structural element that 

 does depend directly upon the environment. This bears out the 

 statement of Shelf ord ('11 a, p. 593) that animal behavior is 

 usually plastic while animal structure is only slightly plastic. 

 Since aside from the size, the taxonomic characters of isopods 

 from the two habitats are the same, the animals cannot be referred 

 to as a stream 'form' as contrasted with a pond 'form,' because in 

 its general usage form is applied to a morphological entity. How- 

 ever, the term 'mores' as used in general in the Concilium Biblio- 

 graphicum, and as specifically defined by Shelf ord ('11, p. 30) 



