214 



THE BIOLOGY OF HYDRA : 1961 



in the feeding reflex in addition to defining the experimental limits 

 within which the ionic composition can be varied. 



Summarized in Table 4 are data concerning the ionic require- 

 ments for the feeding reflex (10). Hydra placed in a 10~^ M 

 solution of the chelating agent ethylenediamine tetraacetic acid 

 (EDTA) lost their ability to respond to glutathione (expt. 1). Since 

 EDTA is known to chelate calcium ion, one of the two environmen- 

 tal cations required for the growth of Hydra, 10~'' M CaCL was 

 added to this same solution, and the Hydra responded normally 

 (expt. 2). No other cation would replace calcium to any degree in 

 reversing the inhibitory action of EDTA except strontium ( expt. 3 ) . 

 Since this metal behaves chemically like calcium, experiment 3 

 strengthens the evidence that calcium is required to effect the 

 feeding reflex. 



Further evidence for the calcium requirement was obtained 



0.4 r- 



--0.2 - 



[CO-H+] 



Molarity x 10 ^) 



Fig. 2. The inhibition of the feeding reflex by magnesium ions, and its 

 reversal by calcium ions. 



