66 University of California Publications in Zoology [Vol. 19 



which Sagitta bipunctata ascends and descends, and why the down- 

 ward migration does not extend deeper than 75 fathoms. 



Since there are no experiments dealing with reactions in water of 

 high or low salinity there does not seem to be any value in discussing 

 the distribution of Sagitta in the sea as regards variations in salinity. 



GENERAL DISCUSSION 



The Effects of Laboratory Conditions in General on 

 Experimental Animals 



In the preceding accounts of responses it was pointed out that there 

 is a noticeable change in the kind of reaction to light in the copepod 

 Calanus finmarehicus and to both light and gravity in Acartia tonsa 

 after the specimens have been kept in the laboratory for a time. To 

 my mind this change is due to something connected with the artificial 

 conditions under which the animals must be maintained, combined 

 with the effect of removal from the ocean. Similar changes have been 

 recorded by other observers in the case of other kinds of organisms. 

 Harper (1905, p. 445) states that specimens of Corethra will collect 

 on the lighted side of a disli when they are first brought in, and that 

 this positive reaction gradually changes to negative. According to 

 Jennings (1904, p. 83) certain rotifers show a like change; and there 

 is a similar statement with regard to the copepod Tcmora longicornis 

 by Loeb (1893, p. 94). Phipps (1915, p. 215) found that there is 

 something associated with captivity that affects the phototropism of 

 amphipods. 



Such observations as have just been mentioned ought to make one 

 question how far reactions that take place under laboratory conditions 

 are likely to indicate the reasons for natural habit. If one is inter- 

 ested in what the specimens at hand are capable of doing under experi- 

 ment it is desirable to allow them to "become accustomed" to labora- 

 tory surroundings. But if experimental results are sought for the 

 information they may give concerning what the animals actually do 

 in nature, it is necessary that the change in behavior be considered. 

 What shall be said, for instance, about such a case as that of Calanus f 

 The animals are positive to light at first, then they become permanently 

 negative at room temperatures. The meaning of such a reversal ought 

 to be considered. It may indicate that the state of the animals has 

 returned to the condition that prevailed at the time of capture, and 



