SPONTANEOUS MOVEMENTS IN LARV.K. 419 



in the rate of respiratory movements which seems always to 

 exist immediately after each spontaneous movement (Chart XV.). 

 At the end of each movement the branchial apparatus is motion- 

 less for a fe\v seconds; it begins activity very slowly, and grad- 

 ually resumes the normal rate in 10 to 15 seconds. This phe- 

 nomenon may be due to acapnia induced by the rush of water 

 through tin- c, r ills while the larva is in motion; furthermore, if it 

 is the production of metabolites which stimulates the body 

 nio\ cment-, ihe same substances would probably increase the 

 volume oi 1'I"<"1 flowing through the branchial vessels and thus 

 favor the \\a-liing out of CO* from the blood; a brief inactivity 

 of tin- re-piratory mechanism then ensues, due to the lack of 



mulation. 



Sprrul.it inn as to the details of the mechanism by which 

 embryonic movements are stimulated and depressed under the 

 \ariou> conditions described in this paper seems premature 

 until the relation between specific effect of CO and variations 

 in the Il-ion concentration has been experimentally determined, 

 diinents with K(\\, with different concentrations of CO 2 , 

 .UK! \\ith ( i.j removed, indicate that essentially similar merh- 

 ani>in- ululate respiratory and body movements, except for 

 >ome . ry reserve system which exists in the case of respi- 



ratory nio\emeiits. 



Thr n-lation of different concentrations of COj to the amount 

 of motility must be a significant factor in determining the 

 miration- and habitat of fishes. Shelford, Wells and Powers, 

 iMiclford. ii their work on the relation of H-ion gradients 



t<> tin- mo\emcnt of fishes apparently did not study the mech- 

 aniMii of thr reactions. The results which these writers report 

 an- pcrhap- due to the direct effects of H-ion changes in the 

 I'o.ly tluids on the primitive neuro-muscular system, or, in otlu-r 

 words, to \ aiiations in the rate of the endogenous, "spontaneous" 

 nio\ ement-> conditioned by alterations in the amount of CO 2 or 

 Il-ioiis in the Mirrounding medium. According to thi> view, the 

 optimum H-ion concentration is automatically dru-rmined }>y 

 the effect of different parts of the acidity gradient on the endo- 

 genous movements, and not to any "choice" or "selection" on 

 the part of the fish. 



A i|iu>tion of broader significance is raised by the>e experi- 



