Light in the Regeneration of Hydroids 139 



PENNARIA TIARELLA 



It has been already pointed out that this hydroid Hves with T. 

 crocea and E. ramosum on the same piles and at the same sea 

 level. Like Eudendrium, it is very much branched, and experi- 

 ments showed that the regeneration of similar pieces from these 

 two hydroids gave practically the same results. 



New hydranths were regenerated in about 48 hours after their 

 removal, and in this regard resembled the other two hydroids. 

 These newly formed hydranths were removed daily, so that the 

 figures actually represent the number of different hydranths regen- 

 erated. 



There is one disturbing factor that has to be reckoned with, 

 namely, the tendency to produce "roots" in place of hydranths 

 particularly after thigmotactic stimuli, such as contact with the 

 side of the dish, or with other stems. In as much as the size of the 

 stalks, of the dishes, the number of stalks in each dish and other 

 conditions were quite the same among the controls and among 

 those in the dark, this disturbing factor may be fairly assumed to 

 be constant in both sets of stalks. In some colonies this ten- 

 dency to root formation is so strong that nearly all cut ends pro- 

 duced roots instead of hydranths. 



The regeneration of Pennaria differs from the other two 

 hydroids in several respects. In the first place, the curve 

 represented by the number of new hydranths on successive days, 

 was not so definite as in Tubularia or Eudendrium, due in all 

 probability to the tendency to heteromorphic "root" formation. 

 Yet the curve of Pennaria approximated very closely that of the 

 other two hydroids. This is shown as follows. The maximum 

 number of hydranths produced on any one day, appeared during 

 the early part of the experiment, and if continued for a long period 

 the number towards the close of the experiment was always very 

 small; and these hydroids, it might be added, were decidedly 

 smaller, i. e., one-half to one-third the normal size. In Experi- 

 ment 5, the number produced on the 2d to the 6th days inclusive, 

 was 12, 15, 16, 13, II hydranths. During succeeding 5 day in- 

 tervals, the greatest number that appeared on any one day was 



