182 NATURE STUDY REVIEW [9:6— Sept., 1913 



about while there. One or two may die in that end from inability 

 to find cooler water. This experiment gives an example of both 

 negative and positive thermotaxis. The animals are negative 

 to the temperature which they avoid, and positive to that in which 

 they collect. 



Discussion. — In case greater extremes of temperature are 

 desired the cold end may be packed in ice on which salt has been 

 sprinkled. When hot and cold tap water are not available, use 

 the same general principle as before but place the lower pans, 

 upon which the experimental pan is to rest, upon ring stands so 

 that sufficient heat (bunsen burner) can be applied to one end to 

 give the desired temperature. Care must be taken to have no 

 shadows in the experimental or control pans as the phototactic 

 response may mask the thermotactic reaction. 



This experiment furnishes an easy method of demonstrating 

 the acclimatization of animals to different temperatures. Ani- 

 mals of the same species collected from the same place, brought 

 into the laboratory and kept in different temperatures will col- 

 lect in decidedly different parts of the temperature gradient. 



Any small water animals may be used in this test though those 

 that rest on the bottom, such as isopods, give most definite 

 results. 



Rheotaxis. 



I. Purpose. — To demonstrate response to an intermittent 

 circular current. 



Material. — ^A granite iron or enamel ware pan ten inches in 

 diameter and about two and a half inches deep. The bottom 

 shoiild be covered with beeswax when crawling animals are to be 

 tested. 



Procedure. — Place the animals in three-fourths inch of the 

 water in which they have been kept. Arrange the light so that 

 there are no shadows in the pan. Usually it is best to use five 

 animals although this number may be varied with the size and 

 condition of the animals. Leave the animals undisturbed for ten 

 to fifteen minutes in order that they may become accustomed to 

 the new conditions and to permit a recovery from the shock of 

 handling. In case they have been kept in a temperature different 

 from that of the room the pan should be placed in a bath that will 

 keep the temperature within one degree of that to which they 

 have been accustomed. 



