252 INVERTEBRATE PHYSIOLOGY 



pressure even when the animal is enclosed within a tight shell. Filtration 

 in such circumstances has been sought and found by Picken (1937). 

 Measurements proved that the hydrostatic pressure of the blood exceeded 

 the colloid osmotic pressure by a substantial margin. By exposing and 

 opening the pericardial sac of Anodonta cygnea, a fresh-water lamelli- 

 branch, he was able to demonstrate a copious flow of a fluid judged to be an 

 ultrafiltrate of the blood, though the exact location of the filtering mem- 

 branes was not identified. The fluid was nearly free of protein, hence was 

 not blood, but otherwise corresponded to blood in its composition. Florkin 

 and Duchateau (1949) confirm the filtration in Anodonta, identifying the 

 ventricle as the site of filtration and finding the same amount of Ca, CI, and 

 PO4 in pericardial fluid and in blood. 



The evidence for filtration as an initial process in urine formation in 

 Anodonta has been further strengthened by the studies of Potts (1954b), 

 who sought to determine the filtration rate by measuring the quantity of 

 inulin lost from the blood stream into the surrounding water per unit of 

 time. It will be noted that Potts made the implicit assumption that inulin is 

 neither secreted nor reabsorbed. The independent active transport of water 

 would not affect the validity of his computations as long as these primary 

 assumptions are correct. From his data Potts computed inulin "clearances" 

 and found that the rate of filtration measured in this way compared 

 favorably with an independent estimate of the rate of urine formation by 

 the technique of placing the animals in an isotonic medium and following 

 the rate of weight loss. It is assumed that salt and water gain or loss will 

 be negligible by any other route than urine formation. The measurements 

 made in this way showed a rate of filtration only about one-fourth that 

 obtained by Picken but, as may be seen from Table 1, a rate still much 

 higher than that for most invertebrate animals. The conditions of Potts' 

 experiments were more physiological than those of Picken's since back 

 pressure due to the presence of an uninjured pericardial sac and a nephro- 

 stome of small dimensions would limit filtration. 



Martin, Stewart, and Harrison (1954) have followed still a different 

 technical procedure in studying filtration in the gastropod, the giant 

 African snail Achatina fnlica. It is possible to chip away the shell of this 

 terrestrial form, exposing the heart and the kidney without blood loss. 

 Fig. 1 shows the relationship of these structures and the points of insertion 

 of small, plastic catheters into pericardial sac, kidney, and kidney ducts. 

 There is the usual renopericardial canal communicating between the peri- 

 cardial sac and the kidney lumen, but repeated attempts to show a sig- 

 nificant amount of filtration into the pericardial sac failed. That filtration 

 was nevertheless taking place, probably through the cell layers of the 

 kidney itself, seems quite clear from the results of the quantitative physio- 



