RHEOTAXIS IN ISOPODA 323 



b. Caffein 



Physiologically, caffein acts as a permanent stimulant. That 

 is, there is no depressing after effect (Cushny, '10, p. 248). For 

 this reason, experiments were run with pond isopods to find if 

 such a stimulant would cause the same increase in the rheotactic 

 reaction as that already produced by increasing the oxygen 

 supply. For the experiments a solution of caffein in distilled 

 water was made, saturated at room temperature, and this was 

 added in small amounts to the ordinary tap water used in the 

 experiments. The amount of caffein is shown in the table (25) 

 in terms of cubic centimeters per liter of water. 



The results of a number of trials are summarized in table 25. 

 The first part compares the effects of different strengths of caffein 

 solution upon the rheotactic response. For the time used, 25 

 cc. of the saturated caffein solution per liter of water proved to be 

 most stimulating although the mortality was high. Part 2 gives 

 a series of trials with a group of pond Aselli exposed to 10 cc. of 

 caffein solution per liter of water. The positive response was 

 increased over 30 per cent and continued high for over eighty 

 hours. The subsequent decrease in the positive response is due 

 to the fact that the isopods apparently became acclimated to the 

 caffein. 



In the third part is given the record of another set of trials with 

 10 cc. of caffein solution per liter of water. Under these condi- 

 tions, the isopods showed a slightly higher positive response and 

 gave the same slump in positiveness at about the same time. 

 These experiments were I'un at the same time as those given in 

 part 2 and the same control was used. Part 4 shows the effect 

 of increasing the caffein after the animals had gone back to their 

 normal reactions, through becoming acclimated to the strength 

 of caffein used. The increase given must be due to the increased 

 amount of caffein, because no other factors were acting. After 

 the acclimatization to this amount of caffein occurred, a further 

 increase served to kill the animals rather than to increase their 

 positive reaction. Again as in the case of the saturated oxygen, 

 the isopods are decidedly more active in all their movements so 



