RESPIRATION 165 



water or by accumulation of possible excretory products. Under condi- 

 tions of oxygen lack, however, the rigour of the outbursts is much reduced. 

 When aerated water is restored after several hours of oxygen deficiency, 

 the worm responds by greatly increased irrigation. Arenicola utilizes 

 30-50 % of the oxygen in the affluent water, and the utilization percentage 

 is not increased when the animal is exposed to low oxygen tension (5 % of 

 normal value), or restored to fully aerated water after prolonged exposure 

 to an oxygen-deficient environment. 



Nereis diversicolor and Chaetopterus variopedatus resemble Arenicola 

 in showing regular cyclical patterns of irrigation activity (Fig. 5.4). In Spiro- 

 graphs spallanzanii pumping movements are nearly continuous under 

 normal conditions; in Sabella pavonina pumping is sometimes interrupted 

 by long quiescent periods. Oxygen deficiency has different effects on these 

 species : Nereis and Sabella respond by decrease or cessation of irrigation ; 

 Chaetopterus by an increase. The determining agent — 2 lack, C0 2 increase 

 or accumulation of harmful metabolites — has not yet been determined. The 

 echiuroid worm Urechis caupo also shows periodical pumping activity, 

 bursts alternating with quiet periods, and about a third of the 2 taken 

 into the hind-gut is utilized. In Nereis virens, utilization of the dissolved 

 oxygen is variable, ranging from 20-75% in water saturated with air. 

 After a period of oxygen lack, ventilation, but not utilization, is increased 

 (28a, 30,75, 100, 158, 159, 161). 



Molluscs. Ventilation in some aquatic molluscs is greatly affected by 

 environmental conditions. In Mya arenaria the ventilation current is 

 increased after a period of oxygen deficiency (low tide). Following a pro- 

 longed period of anaerobiosis (21 hours), normal levels are not restored 

 until 3-4 hours later (Fig. 4.14). The utilization of oxygen is normally low, 

 between 3-10%, but after a period of low tide when the animal has con- 

 tracted an oxygen debt it rises appreciably (to about 25%). In lamelli- 

 branchs the flow of water over the gills shows little variation when the 

 valves are open. When submerged, Mytilus edulis keeps its valves open 

 most of the time and pumps almost constantly. In other bivalves the valves 

 are periodically closed and ventilation ceases. The oyster (Ostrea virginica), 

 for example, closes its shell, on the average, 7 hours out of 24 (29, 32, 55, 

 87, 88). 



The differential effects of 2 decrease and C0 2 increase have been dis- 

 tinguished in several species. Ventilation in the oyster {Ostrea virginica) is 

 affected by acidity. When HC1 is added to sea water so as to lower the pH 

 to 7-0-6-75, the oyster responds by increased pumping activity. In con- 

 formity with their more complex organization and physiology, cephalopods 

 show a high degree of respiratory control. Oxygen lack in Octopus causes 

 increased ventilation up to ten times normal. Increased C0 2 at tensions 

 below 6 mm also produces an increase, but higher tensions inhibit respira- 

 tion. Oxygen utilization varies between 50-80% (76). 



Crustacea and Xiphosura. Respiratory control shows much variation in 

 different groups of crustaceans. The respiratory (and feeding) movements 



