l8O MAX MORSE. 



In another set of experiments, excess of oxygen was supplied 

 by means of the Mast aerating device. The vessel containing the 

 hydroid stems was placed in chipped ice and the aerating appa- 

 ratus delivered bubbles of air to one corner of the dish. How- 

 ever, here, no lengthening of the period of retention of the 

 hydranths could be determined when the temperature was per- 

 mitted to rise to ordinary room temperature (21 C.). There- 

 fore, while lack of oxygen causes autotomy to set in, an excess 

 of the gas over and above that which forms normally the oxygen 

 content of sea water at these temperatures does not prolong the 

 period of retention of the hydranths ; they behave as if they had 

 been brought in from the sea and placed in an ordinary aquarium 

 under ordinary conditions. 



It will be recalled that the specimens in the fingerbowls which 

 were kept in the ice-chest were not supplied with air artificially. 

 Moreover, the water was not changed, so that the air which 

 was over the surface of the water of the bowls was sufficient 

 to supply the oxygen needed by the animals. The oxygen con- 

 tent of water kept under such conditions cannot vary to any 

 great degree and the sea water, flowing over the animals in their 

 natural habitats varies, as has been determined, comparatively 

 little in the amount of air in solution. Moreover, the specimens 

 which, on being introduced into the laboratory lose their 

 hydranths, are kept, in the main, supplied with ever changing 

 water in which the oxygen content is practically that of the 

 surrounding sea, or even higher, owing to the condensation by 

 the pumps. Therefore this factor cannot be of any considerable 

 importance in causing autotomy. 



5. Considerations were given to mechanical effects such as the 

 swaying of hydranths on the stems in a strong current of water, 

 rubbing of the stems against one another and mechanical shock 

 induced by cutting off the hydranths from the stems. 



With respect to the first factor, we know that in nature, in 

 many cases, Tubnlaria grows where tide causes whirlpools and 

 an ever changing direction in the currents passing over the 

 hydroids and therefore, one would not look for this as a cause 

 of autotomy. Experiments were performed to show that the 

 factor is not operative, by placing stems, bearing healthy 



