Light in the Regeneration of Hydroids I41 



tion could be induced with far more certainty by exposures of one 

 hour or one-half hour for several successive days. And although 

 very few hydranths were produced at any one time they appeared 

 on as many as 5 separate days. Thus, daily half hour exposures 

 resulted in 0, 0, 3, 6, o, hydranths in one set of stalks and o, o, 

 I, I, I, o in another set. One hour exposures daily gave rise to 

 o, o, o, I, I, o and o, o, o, 4, o, o, and o, o, 2, i, 2, 2 hydranths 

 in 3 different sets of stalks. 



If left in the light for a whole day, hydranths were almost certain 

 to appear in every experiment and these were large and numer- 

 ous. Though the absence of light was inimical to development 

 yet no permanent injury resulted from prolonged retention in the 

 dark. For on returning the stalks to the light many normal 

 sized hydranths were immediately regenerated. After 10 days 

 in the dark the stalks in Experiment 5, were brought into the light 

 and produced o, i, 4, 8, o, 5, 5, i, o, 3, o, 2, o, 4, i, 3 hydranths 

 during the next 16 days. In Experiment 6, after 16 days in the 

 dark o, 14, 31, 26, 68, 17, etc., hydranths were regenerated on 

 successive days. 



There is a variety of Pennaria that is found attached to eel grass 

 not far from shore. This is said to belong to the same species as 

 that found on the much more shaded piles of the wharves, though 

 it differs from it in several respects. Some experiments were 

 made upon this variety, to determine whether the difference in 

 habitat was reflected in a difference in the amount of light required 

 to stimulate regeneration. The stalks of this variety, it was found, 

 required as a rule longer periods of exposure to stimulate the 

 development of new hydranths. In other respects their behavior 

 was quite the same. 



Loeb- in his account of the regeneration of Eijdendrium ramo- 

 sum stated that this hydroid never regenerated in the absence of 

 light. He undoubtedly confused Eudendrium with Pennaria tiar- 

 ella. In conversation with the writer Loeb expressed the opinion 

 that this was probably the case. 



These comparative studies "make it perfectly clear that so re- 

 markable a sensitivenes as that displayed by Eudendrium (after 

 the first cycle) finds no parallel among the other two closely 



