HEAD-FREQUENCY IN PLAN ARIA 411 



in the preceding section that amount of available nutritive ma- 

 terial is not the primary factor in determining head-frequency. 

 Further discussion is postponed to the final section of the paper. 



HEAD-FREQUENCY IN RELATION TO MOTOR ACTIVITY OF PIECES 



These experiments consist essentially in determination of head- 

 frequency in pieces which are repeatedly stimulated to motor 

 activity during a longer or shorter time beginning immediately 

 after section (II) as compared with that of smaller pieces which 

 are left undisturbed (I). The results for two series are given in 

 table 3. In both cases the pieces B and C (fig. 1) are used be- 

 cause such pieces have neither an extremely high nor an extremely 

 low head-frequency and change in both directions is readily 

 possible. In A pieces and in pieces from the posterior zooids the 

 head-frequency is usually so high that only decrease appears 

 clearly in experiment. The methods of stimulation were various : 

 currents of water were used to loosen the pieces from the glass, 

 some motor activity usually following; individual pieces were 

 loosened and turned over with the aid of a camel's-hair brush and 

 usually attempted sooner or later to turn back again. Gentle 

 stroking or slight pressure with a camel's-hair brush was also 

 found to be effective. In series 302 such stimulation was re- 

 peated at least every hour for some eight hours after section, 

 after which the pieces were left undisturbed overnight. On each 

 day following until the new heads and eyes were distinctly visible 

 the pieces were stimulated at least once an hour from 8.30 a.m. to 

 6 p.m. and at least twice between 8 and 11 p.m. In order to 

 equalize oxygen supply as far as possible the water on the un- 

 stimulated lots (I) was stirred gently at the times when the other 

 lots were stimulated, the stirring being usually not sufficient to 

 induce motor activity. 



In series 498 the stimulation of II was repeated every five to 

 ten minutes during the first two hours after section, once an hour 

 during the second two hours, and again every five to ten minutes 

 during the third two hours. After this the pieces remained 

 undisturbed. It was known at the time of this experiment that 

 the continuation of the stimulation during several days as in 



THE JOXJBNAl. OP EXTERIMENTAL ZOOLOGY, VOL. 30, NO. 3 



