356 THE BIOLOGY OF HYDRA : 1961 



whereas cultures grown in the bottoms of dishes quickly 

 become necrotic." (6). This striking "position effect" in 

 CordylopJwra contrasts with Hydra which can grow 

 equally well on the bottoms or sides of dishes. Preliminary 

 results suggest that growth of Cordylophora is especially 

 sensitive to the self-induced acidity present in the halo 

 zone and that such zones are largely prevented from 

 forming on slide-grown cultures by thennal currents 

 ( these can be made visible with methylene blue or other 

 dyes ) . Experimental elevation of the pCOa in shaken cul- 

 tures gradually inhibits Cordylophora growth whenever it 

 is sufficient to lower their pH below about 6.7, the actual 

 pCO:- varying with the buffering capacity of the solution 

 employed. 



4. Strobilization in Cyanca arctica. 



This organism buds indefinitely when fed and placed in 

 clean water daily. One culture strobilized and produced 

 many medusae after being left untouched for a month in 

 an ice-box at 12'. 



5. Spiral persons in Hydractinia and Podocoryne. 



Braverman has shown that spiral zooids of Podocoryne 

 appear on the rim of hermit crab shells only if the shell 

 is inhabited by a living hermit crab (4). He noted that 

 spiral zooids never form on colonies grown on glass slides 

 in the laboratory, an observation that we have confirmed 

 on Hydractinia. Spiral zooids appear all over slide- 

 grown Hydractinia cultures exposed to a pCO^ of 2% 

 atm.: a result that suggests that CO2 coming from the 

 respiration of the hermit crab is the stimulus that creates 

 spiral zooids on lips of hermit crab shells. 



6. Parthenogenetic reproduction in Daphnia. 



Both Daphnia longispina and Daphnia magna fail to re- 

 produce parthenogenetically in aerated water while doing 

 so in water whose pCO:.. is 1% atm. and whose pOo is 5% 

 atm. Daphnia are thus neither aerobic nor anaerobic 

 organisms, but like microaerophilic ("little air") bacteria, 

 they require partial anaerobiosis to live. This fact ex- 



