288 



THE BIOLOGY OF HYDRA : 1961 



neither light nor shght variations in temperature are critical. These 

 standard conditions ( 1 ) have been used for all the experiments dis- 

 cussed here, since variation of the conditions leads to alterations in 

 colony pattern. 



The number of hydranths in a Cordylophora colony increases 

 exponentially with time in the beaker-slide cultures, as do the hy- 

 dranths of Ilydia in Loomis cidtures. It is thus possible to compute 

 the growth rate of this colonial organism, using standard equa- 

 tions for exponential growth. This growth rate has been used to eval- 

 uate the growth conditions described above. Cordylophora colonies 

 double about every three days, or more slowly than Hydra littoralis, 

 which doubles in less than two days. The fact that Cordylophora 

 colonies grow exponentially even though they are colonial is ol 

 interest and we shall return to it later. 



stolon lip 



Figure 1. Diagram illustrating the basic pattern and macroscopic features 

 of a Cordylophora colony. Sketched from a photograph of a laboratory colony. 



This culture method provided uniform Cordylophora colonies 

 with which I could begin to study colony fomiation. Time-lapse 

 movies taken to study the growth of colonies revealed a markedly 

 organized system of peristaltic waves, which probably act to circu- 

 late nutrients through the colonies.- These waves are proximally 

 oriented, beginning at the tip of each hydranth and passing down 



-A movie demonstrating the features of peristalsis in Cordylophora was shown at tlie 

 meeting. The apparent synchrony of peristalsis is still being studied. 



