RESPIRATION IN WATER 43 



other purposes along with respiration (bivalves, phyllopods, 

 and others). In Ligia oceanica Fox and Johnson (1934) found 

 the beating of pleopods, which produce the ventilating current 

 for the gills proper, unaffected by O2 except at very high and 

 very low (< 2 ml 1) concentrations which both caused a 

 slowing of the rhythm. 



In Gammarus locusta they found that the pleopods need not 

 move at concentrations above 5.6 ml/1. A fall in concentra- 

 tion caused a rapid beating which, however, slowed down 

 again— even to 0— in 15 minutes with unaltered concentration. 



In the higher Crustacea, Cephalopoda, and fishes there is no 

 doubt whatever that oxygen lack stimulates respiration 

 (Westerlund, 1906; Heerdt u. Krijgsman, 1939), while the 

 effect of CO2 is sometimes doubtful and never very pronounced 

 (Olthoff, 1934; Helga Meyer, 1935). The normal C0 2 ten- 

 sion of natural waters is almost always very low, and C0 2 is 

 certainly not a regular stimulant. It is contended (Winter- 

 stein, 1911, 1921; Olthoff, 1934) that oxygen lack may act by 

 inducing an acid reaction in the "respiratory center," but this 

 idea is purely hypothetical. 



