RHYTHMIC PULSATION 351 



NH4OH did this for about 15 hours. The amplitude of pulsa- 

 tion in these mixtures was greater than in controls run in sea 

 water. It is suggested that alkalis favored the continuance of 

 rhythm by assisting in ionic exchanges at the surface of the con- 

 tractile elements; probably this was accomplished in part in a 

 secondary way, namely, by the neutralization of acid metabolic 

 products. 



c) Hydrogen-ion concentration. The hydrogen-ion concentra- 

 tion of sea water is given by various authors as lying between 

 0.5 X 10-« and 1.5 X 10"^ (Hober, '14, p. 195). The sea water 

 used in experiments with Stichopus was faintly alkaline to neu- 

 tral red and neutral to tropaolin ''000." Its Ch- was thus about 

 10-^ Increasing the Ch. by the addition of acid (HCl)^^ led to 

 the rapid cessation of pulsation movements. 



Experiment 95.2. July 24. Sea water to which tb- HCl had been 

 added until just neutral to Congo red (i.e., Ch- = ca. 10~'**^) was tested 

 with five fresh cloacal preparations. Pulsations of an irregular char- 

 acter were manifested for about 15 minutes in two of these tests. The 

 anal sphincter did not pulsate, but remained about one-third closed. 



In transferring the cloacal preparations to a desired solution, 

 they were lifted out of sea water; they thereupon ceased beat- 

 ing (contracting as the result of mechanical stimulation) ^^ and 

 if returned to normal sea water about two minutes usually 

 elapsed before pulsation began again. It was sought to utilize 

 this fact and control the hydrogen-ion effect more closely (as 

 was done by Dale and Thacker, '14, in analyzing the automaticity 

 of different regions of the frog heart) by discovering the concen- 

 tration which would just permit pulsation to begin and continue 

 for about a minute. For the sphincter this concentration was 

 attained by mixing equal volumes of sea water and sea water 

 made just barely acid to Congo red: the mixture had therefore 

 a Ch- of ca. 10"^ This limit is very close to that found by Bethe 

 ('09, p. 261) for the pulsations of medusae. 



1^ The effect of certain organic acids was also studied, but not with sufficient 

 completeness to warrant discussion in this place. 



1"-' No pulsations were ever observed out of sea water. 



