66 SURVEY OF INVERTEBRATES 



permitting its metabolic activities to be carried on during 

 the period of low tide. The Arenicola haemoglobin has 

 a steep dissociation curve, thus enabling the worm to 

 secure oxygen even at rather low tensions. The amount of 

 oxygen present in the blood, when the latter is saturated, 

 was found to last over a period of 71 minutes. These 

 factors combined may suffice to prevent the necessity of 

 a predominantly anaerobic life during low tide. 



The same conditions may be assumed in the case of the 

 gephyrean Urechis caupo. Redfield and Florkin (1931) 

 calculated that the oxygen present in the blood of this 

 animal would be sufficient to last for about 3 hours. The 

 bottom in which the worms build up their tubes may fre- 

 quently be out of water for periods extending to 6 hours, 

 occasionally to 18 hours. The oxygen content of the 

 water present in the tubes, however, never decreases to 

 less than 0.6 cc. per liter and, if the ground becomes drier, 

 somewhat higher values are found. At these tensions 

 the blood is still saturated to 60 per cent with oxygen 

 which should allow a sufficient supply for the tissues. 

 MacGinitie (1935) observed furthermore that burrow- 

 ing animals of this habitat (amongst them Urechis) cease 

 all activity at low tide. This, naturally, lowers their oxy- 

 gen requirements. If they are placed in oxygen-free 

 water, they become entirely quiet and ''scarcely can be 

 prodded with a glass rod sufficiently to make them show 

 activity. ' ' 



Whether planktonic marine worms must occasionally 

 supplement their aerobic metabolism through anaerobic 

 processes is not yet clear, although it is known that some 

 of them penetrate readily into strata quite poor in oxy- 

 gen. According to Nikitin (1931) some unspecified poly- 

 chaete larvae, and less frequently Sagitta euxina, are 

 found in the deeper layers of the Black Sea where the 

 oxygen concentration reaches only 2 to 5 per cent of the 

 saturation value. 



