142 A- J- Goldfarh 



associated hydroids. On the other hand the absence of hght is 

 not so directly preventive of hydranth formation among Tubularia 

 and Eudendrium as in Pennaria. These hydroids Hving practi- 

 cally in the same environment agree in that after they have 

 ceased to produce hydranths they may be stimulated to regener- 

 ate them by light and vice versa its absence retards and ulti- 

 mately inhibits development. But the conditions and the de- 

 gree to which light is effective varies with each hydroid. In 

 Parypha and Eudendrium the absence of light inhibits regenera- 

 tion only after a prolonged preliminary period. In Pennaria it 

 is a coditio sine qua non from the very beginning. 



SUMMARY 



Light is a well defined factor in the regeneration of these 

 hydroids, but the degree of effectiveness and the duration ot the 

 preliminary period required to render the hydroids susceptible to 

 light stimuli varies. Eudendrium ramosum has a long pre- 

 liminary cycle during which regeneration takes place in the 

 dark almost as well as in the light. After this period, no 

 regeneration occurs so long as stalks are mamtamed in the 

 dark. A very brief stimulus, that is, an exposure to the light 

 of 15 seconds, sufficed to call forth a series or cycle of hydranths. 

 New series of hydranths could be produced again and again by 

 repeated exposures. Like Eudendrium, Tubularia crocea also 

 has a preliminary period of about 13 days during which hydranths 

 are developed almost as well as in the light. At the expiration of 

 this cycle, regeneration may be stimulated by exposure to the 

 light of about 15 or more minutes. Pennaria tiarella differs from 

 the other two hydroids in that there is no preliminary cycle. From 

 the beginning hydranths are never regenerated in the dark. They 

 may be stimulated to develop only by long exposures of 2 hours or 

 more, or by exposures of one-half to one hour daily. 



