36 



RESPIRATORY MECHANISMS 



depending upon the temperature and the oxygen content of 

 the water. At low oxygen contents (2 ml/1) the ventilation 

 may become continuous, but at very low (0.6 ml/1) it may stop 

 almost completely. The utilization of the dissolved oxygen 

 is variable, but may reach the high value of 70%. 



Arenicola was studied by van Dam, who found that the 

 animal would respire in a normal manner when enclosed in a 

 U-shaped glass tube as shown in Fig. 11. It is characteristic 



Fig. 11. Determination of ventilation volume in Arenicola. (van Dam.) 



also for Arenicola, that breathing is intermittent. In one 

 typical case there were pauses of about 20 minutes (at 20°C) 

 and activity periods of about 10 minutes, during which about 

 90 ml water were passed through the tube. Thirty to 50% 

 of the oxygen in this water was normally utilized by the ani- 

 mal, and it is a very interesting fact that the utilization per- 

 centage remained about the same, also when the water con- 

 tained only 5% of the normal amount of oxygen or when the 

 animal breathed normal water after a prolonged exposure to 

 low oxygen. 



In the decapod Crustacea the gill cavities below the carapace 

 on both sides of the body are ventilated by the regular beating 



