Millimeters 



0.5 



Figure 247. — Section through the reservoir and renal duct of the right nephridium C. virginica. Gonad at the lower left 

 corner and cerebro-visceral connective at the upper right corner. Bouin, hematoxylin-eosin. 



The experimental procedures for separate collec- 

 tion of blood, pericardial fluid, and urine from the 

 reservoir are uncertain, both because the tissues 

 are fragile and because a rich supply of blood is 

 received by the nephridia directly from the visceral 

 sinuses. Prosser (1950, p. 36) states correctly that 

 urine volume cannot, therefore, be calculated from 

 the rate of accumulation of the pericardial fluid ; 

 neither can the composition of urine be inferred 

 from an analysis of the fluid collected from the 

 interior of the kidney. 



The rate of filtration of blood from the ventricu- 

 lar wall into the pericardium was studied in Ami- 

 (lonta by Picken (1937) using the following method : 

 A hypodermic needle of 1.5 mm. bore was inserted 

 in the pericardial wall previously exposed by 

 cutting off the adjacent portion of the shell. A 

 \ length of about 2 cm. of glass tubing was sealed 



THE EXCRETORY SYSTEM 



to the hypodermic needle, which was tilted so 

 that there was a difference of about 0.5 cm. in 

 the levels at each end of the needle. The fluid 

 from the pericardium dropping through the needle 

 was collected for periods of 5 minutes at 10-minute 

 intervals. The average rate of filtration deter- 

 mined in this manner was about 25 ml. in 4 hours. 

 The method probably gives a higher rate of blood 

 filtration than occurs under normal conditions be- 

 cause of a loss of pressure inside the pericardium 

 resulting from the punctm'e of the wall. Fm'ther- 

 more, it does not necessarily follow that the rate 

 of the discharge of fluid by the kidney is con- 

 trolled by the rate at which the pericardium fills 

 up with blood. The method does not seem ap- 

 plicable to the oyster because of the position of 

 the excretory organs on the oyster body and the 

 inherent technical difficulties. 



275 



