412 HEXRY C. TRACY. 



the movements were immediately stimulated to such an extent 

 that they were nearly continuous during the first 15 minutes; 

 they slowed down gradually, reaching about normal at the end 

 of the first hour; during the next following 15 minutes, the 

 movements ceased entirely. The results of these experiments 

 show a close resemblance to reactions which would have been 

 expected from a respiratory mechanism under similar con- 

 ditions. 



During the summer of 1923 at the laboratory of the United 

 States Bureau of Fisheries, 1 at Woods Hole, I attempted to 

 carry out more accurate experiments by subjecting Opsamis 

 larvae to sea water of known definite partial pressures of CO 2 . 

 For suggesting this method I gratefully acknowledge my in- 

 debtedness to Doctor Homer W. Smith. Details of the method 

 have since been published (Smith and Clowes, '24). 



METHODS. 



The method of experiment consisted essentially in adding 

 different percentages of HCL n/2O to the sea w r ater in which the 

 larvae were placed. The changes in CO2 partial pressure, and 

 the H-ion concentration produced by this means are stated in 

 the tables. 



In applying this method, the following sources of error had 

 to be taken into account: 



(1) Variation in frequency and duration of the spontaneous 



movements in different individuals, and in the same 

 individual at different times. 



(2) Temperature changes. 



(3) Stimulation by manipulation and handling of the embryo 



previous to the beginning of each experiment; previous 

 observations had shown that manipulation of the yolk 

 sac which results from handling of the larvae, pressure 

 of flowing water, etc., may cause an increased movement 

 even though the larvae do not respond to direct tactile 

 stimulation. 



1 In connection with laboratory arrangements in the conduct of these experi- 

 ments I am greatly indebted to Dr. R. E. Coker, Director of the Laboratories of 

 the Woods Hole Station of the II. S. Bureau of Fisheries. 



