RESPIRATION IN WATER 35 



It is possible in some cases to measure the ciliary flow of 

 water, and in most cases samples can be drawn from the 

 incoming and outflowing water, and the oxygen content of 

 these samples compared. This affords a measure of the 

 respiratory efficiency, which is the higher the more oxygen is 

 extracted from the water by the gills. The percentage of 2 

 removed is called the utilization. 



Van Dam (1935, 1938) studied the utilization of oxygen in 

 the clam, Mya arenaria. Normally the utilization is very low, 

 varying between 3 and 10%, but just after low tide when the 

 animal has contracted an "oxygen debt" it may rise to 35%. 

 The absolute utilization in one animal was found to be inde- 

 pendent of the oxygen content of the water, amounting to 

 0.27 ml per liter water whether this water contained 15 ml/1 

 or only 2 ml/1. In Anodonta a. utilization exceeding 90% could 

 be provoked by leaving the animals dry for 5-48 hours. 

 Hazelhoff (1935) has observed utilizations of 7% on an average 

 in Lamellibranchia and 6% in Ascidia. In all these animals the 

 flow of water along the gills varies but little (at a constant 

 temperature), but may be suspended altogether for some time. 



Several marine worms construct U-formed tubes in the 

 bottom material in which they live, and pump water through 

 these tubes both for feeding and for respiration. Although 

 their gills are external they are well protected and ventilated 

 by muscular activity. 



Nereis virens lives in shallow water and often in the intertidal 

 zone in U-shaped burrows, from which it comes out for food. 

 The animal pumps water at intervals through the burrow, 

 and it is a significant observation of A. Lindroth (1938), who 

 has carefully studied the respiration in the natural burrows, 

 that the presence of a trace of mussel juice in the water, or a 

 small piece of mussel meat placed outside, causes the pumping 

 to be greatly increased and may call the animal forth from its 

 burrow. The ventilation therefore serves both the purpose 

 of smelling food and the purpose of respiration. When not 

 disturbed, Nereis ventilates the burrow at varying intervals 



