114 C. M. CHILD. 



colony, i.e., the terminal hydranth of the main axis, although 

 the oldest of all in point of time, and the terminal hydranths 

 of the more apical branches, which are, as regards time, the 

 youngest terminal hydranths, are on the average more susceptible 

 than terminal hydranths of more basal branches. For example, 

 characteristic cases show 60-80 per cent, of the fully developed 

 terminal hydranths of the apical half of a colony disintegrating 

 at a time when 60-80 per cent, of the terminal hydranths of the 

 basal half are still intact. In any well developed primary branch 

 bearing secondary branches, the susceptibility differences in 

 the terminal hydranths of the secondary branches are in general 

 similar to those in the primary terminal hydranths. The termi- 

 nal hydranth of the primary branch and the hydranths of the 

 more apical secondary branches are on the average more sus- 

 ceptible than those of more basal secondary branches. In short, 

 the colony as a whole and each larger branch shows definite and 

 orderly susceptibility relations among its hydranths, and these 

 relations are similar to those observed in algae with similar 

 growth form. Bougainvillea shows similar susceptibility rela- 

 tions, but as might be expected from the less regular form of 

 colony they are less distinct and uniform than in Pennaria. It 

 is perhaps necessary to emphasize the point that these are general 

 and average differences and are best determined by comparing 

 percentages of terminal hydranths in process of disintegration at 

 a given time in different regions of the colony, rather than by 

 comparing individual hydranths. The differences are not as 

 great as the differences with stage of development and are clearly 

 brought out only by some statistical method. Even the apical 

 hydranth of the colony does not always disintegrate earlier than 

 any other fully developed hydranth, but it does do so in a large 

 proportion of the cases in fresh material and the relative suscepti- 

 bility of the other terminal hydranths is likewise not absolutely 

 uniform. In fact this relation in the hydroid is much less distinct 

 and uniform than in many of the algae. 



It is a fact of some interest that in Pennaria and Bougainvillea 

 the apical hydranth of the colony is the most susceptible or 

 among the most susceptible of the fully developed hydranths, 

 although the oldest in point of time, and that the terminal 



