62 C. M. CHILD. 



and the age differences appeared in all with the few exceptions 

 above noted. One of these exceptions was rather remarkable. 

 In KNC W/5OOOO animals 7-8 mm. usually lose the ability to 

 pulsate, even on stimulation, in 3-10 minutes, animals I5mm. after 

 1-2 hours, while in w/ioooo the times for cessation of pulsation 

 are respectively 1-2 minutes and 3-10 minutes. In one case, 

 however, in KNC m/ioooo an animal 15 mm. ceased to pulsate 

 in three minutes, while an animal 8 mm. in the same closed con- 

 tainer continued normal pulsation during thirty minutes, longer 

 observation being impossible. For some reason this particular 

 animal was practically insusceptible to KNC w/ioooo, a concen- 

 tration which stops pulsation in the younger individuals of the 

 other species examined in 5 minutes or less and in other younger 

 individuals of Sarsia in 13 minutes. This extreme exception is 

 merely recorded without any attempt to account for it. The 

 observations on Sarsia do indicate a wider range of physiological 

 condition in animals of a given size than in the other species 

 examined, but they afford no clue to interpretation of a condition 

 so extremely exceptional as this. 



CONCLUSION. 



The preceding data make it evident that the hydromedusa, like 

 other animals, undergoes a progressive change in physiological 

 condition with advancing development, the differences in beha- 

 vior and susceptibility being indicators of this change. It has 

 been shown that in other forms a decrease in the rate of oxida- 

 tions is a characteristic feature of the change and the facts indi- 

 cate that a similar decrease occurs here, although these forms are 

 not very favorable material for the direct determination of 

 oxygen consumption or CO 2 -production. The assumption that 

 the differences in susceptibility are due merely to differences in 

 permeability of surface membranes is refuted by the results ob- 

 tained with neutral red and methylene blue and by the cases of 

 acclimation. Moreover, there is every reason to believe that 

 the age differences in susceptibility to cyanides are at least more 

 directly associated with the oxidation rate than with permeability. 



As regards permeability, however, it is becoming more and 

 more evident to investigators in this field that the condition of 



