AUTOTOMY OF HYDRANTH OF TUBULARIA. 179 



for more than a few moments during the day. Therefore, if 

 the absence of illumination was a positive factor in inducing the 

 process of autotomy, it most assuredly would have been made 

 evident here. In another case, specimens were attached to 

 weighted blocks of wood, as used in an earlier experiment, the 

 block being covered with a tin case. These were lowered be- 

 neath a bridge over a tide-way to a depth of two fathoms. The 

 individuals were therefore in almost total darkness. The tem- 

 perature of the water at this depth was lower than that of the 

 surface, owing to tide currents. The hydranths were retained 

 for over two weeks, when the experiment was terminated. 



3. Stems of Tubularia were tied to supports in running water 

 of a temperature of about 20 C. The supports could be so 

 arranged that the stems were inclined at any angle with respect 

 to gravity. The hydranths were cut off by autotomy in three 

 days at whatever angle the stems were placed. Inasmuch as this 

 time corresponds to that in which the hydroids of colonies intro- 

 duced into aquaria from the sea, lose their hydranths and since 

 no decrease in the time of autotomy could be determined, to be 

 correlated with the position of the hydroid with respect to 

 gravity, the writer does not believe that this factor is operative 

 in autotomy. Loeb, it will be recalled (/. c.\ believed that 

 gravity induced absorption in Antennularia. This case of Loeb's 

 is the only one recorded in which gravity acts. When one recalls 

 the fact that the stems of Tubular la are placed in any position 

 with respect to the vertical in their natural habitat on rocks and 

 piles, it would seem quite extraordinary that such effects of 

 gravity should obtain in this species. Moreover, in strong cur- 

 rents, such as the tide which sweeps over the rocks of the Tide- 

 mill at South Harpswell, Maine, the stems are subjected to a 

 force which causes incessant change in their axes with respect to 

 gravity. 



4. To determine the effect of lack of oxygen, a liter of sea 

 water was boiled for half an hour and cooled to 20 C. in an 

 hermetically sealed vessel. Specimens of Tubularia were put 

 into the water and the vessel, now exposed to the air, was placed 

 in the ice-chest. Within two hours nearly every hydranth had 

 fallen from the stems. 



