SUSCEPTIBILITY IN HYDROIDS. 123 



those zooids which are most susceptible to the lethal concentra- 

 tions, show the greatest capacity for acclimation to a certain 

 range of low concentrations. 



CONCLUSION AND SUMMARY. 



The significant points in the experimental data are; first, 

 that the susceptibility relations are definite, not merely for the 

 single tentacle, hydranth and medusa bud, but for the single 

 axis as a whole and for the axial complexes which result from 

 branching and finally for the whole colony; second that they are 

 similar to the relations observed in other organisms with similar 

 growth forms, e.g., the algae. In general, the more apical regions 

 of the physiologically younger axes of a complex, which are 

 obviously the regions of most intense metabolism, are most 

 susceptible to the higher concentrations or intensities and also 

 most capable of becoming acclimated to slightly inhibiting con- 

 ditions. These differences in susceptibility and capacity for 

 acclimation are manifestly associated with quantitative differ- 

 ences in physiological condition, which are fundamental charac- 

 teristics of physiological axes. The similarity of susceptibility 

 relations in algae and hydroids of similar growth form, indicates, 

 if it does not demonstrate their independence of the specific 

 differences in algal and hydroid protoplasms and therefore their 

 essentially quantitative character. In spite of the differences in 

 rate of staining with vital dyes, it is evident that differences in 

 susceptibility cannot depend solely upon differences in permeabi- 

 lity of limiting surfaces, though such differences in permeability 

 are doubtless associated with the conditions on which suscepti- 

 bility depends. Regional or axial differences in permeability 

 are themselves dependent upon the fact that the membranes are 

 living and therefore metabolically active, and upon the possi- 

 bility of quantitative differences in this living condition. Some 

 substances enter cells only as they produce irreversible changes 

 in the living surface, while others enter all cells readily without 

 appreciable injury to the surfaces, yet to both groups of sub- 

 stances the axial and regional differences in susceptibility are in 

 general similar. 



These data on susceptibility in the hydroids are in complete 



