SUSCEPTIBILITY IN HYDROIDS. 113 



placed and becoming the tip of a lateral branch (Fig. 10). 

 In these forms what is apparently the main axis is actually made 

 up of a portion of each successive axis formed. In the monopo- 

 dial Pennaria the apical zooid is, except for injuries or losses, 

 the first zooid developed and so the oldest in point of time of 

 that stem or branch, while in Obelia the apical zooid is the latest 

 developed and the youngest in point of time, except for the new 

 bud which is soon to take its place. 



In both types, however, certain characteristic relations exist 

 between main axes, real or apparent, and lateral branches. In 

 Pennaria and the Obelia species examined, for example, lateral 

 branches grow more slowly than the main axis, so that the 

 colony developing without injury or crowding shows more or 

 less the general form of a conifer. It has been experimentally 

 determined that in plants of such form very definite physiological 

 relations exist between main and lateral axes, and it has been 

 possible to show for various algae of these growth forms that the 

 susceptibility relations are likewise very definite in character 

 (Child, 'i6a, b, '17). The similarity of growth form in plants 

 and hydroids indicates that the physiological relations between 

 the constituent members of the complex must be in certain re- 

 spects similar and therefore quite independent of the specific 

 differences in plant and animal protoplasm, in other words quanti- 

 tative in character. 



The susceptibility of the hydranths of different regions of the 

 whole colony or of particular stems or branches shows certain 

 characteristic differences which are similar to those found in the 

 algae and associated in a definite way with the growth relations 

 of the constituent axes. In a well developed colony of Pennaria 

 all primary branches, except those latest formed just below the 

 apical end of the main axis, terminate in large hydranths of 

 about the same developmental stage, so far as appearance is 

 concerned, though of course, where losses and regenerations have 

 not occurred, the apical hydranth of the main axis is the oldest 

 in point of time, that of the most basal branch the next oldest 

 and that of each successive branch successively younger. In 

 fresh material, however, the susceptibility of these full grown 

 terminal hydranths decreases in general basipetally in the 



