208 THE BIOLOGY OF HYDRA : 1961 



thione for the glutathione-receptor (13) (Table 1, expt. f), some 

 Hydra took as long as 6 niiniites to open their mouths, while others 

 did not carry out the feeding reflex at all. In these cases, the stand- 

 ard deviation is large relative to tr-ti. Data of this type are similarly 

 expressed as the average time (tf-ti) during which the mouths of 

 the five Hydra tested remained open regardless of the number that 

 responded positively. 



TABLE 1 

 Method of expressing the duration of the feeding reflex 



Glutathione t^-tj ( min. ) 



Expt. concentration tj(min. ) tf(min.) Mean rt S.D. 



Hydra starved for two days were used in all experiments. 

 Data from reference 8. 



The values for tr-ti at excess glutathione concentrations are 

 usually within the range of 25 or 35 minutes, depending upon 

 whether the Hydra were starved one or two days preceding the 

 experiment. This fact should be borne in mind when comparing 

 data from different sets of experiments. ( It now also is known that 

 small changes in temperature influence tf-ti significantly as shown 

 in Table 7. ) 



These data, in addition to providing a basis for the assay, give 

 insight into central problems concerning the mechanism by which 

 glutathione elicits the feeding reflex. The values given for ti must 

 include the time required for at least two major processes to occur: 

 ( a ) the union of glutathione with its receptor, which in experiments 

 a-d is probably rapid (i.e., within a few seconds), and (b) all 



