RENAL FUNCTION 263 



more primitive phyla. After reviewing a list including Platyhelminthes, 

 Rotifera, Nemertea, Acanthocephala, Priapuloidea, Entoprocta, Gas- 

 trotricha, Kinorhyncha, Cephalochorda, some Archiannelida and Poly- 

 chaeta, and larval stages of Polychaeta, Archiannelida, Echiuroidea, Mol- 

 lusca, Phoronidea, and Cephalochorda, Goodrich (1945) says: "These 

 organs (protonephridia) are in fact widely distributed and may be inferred 

 to have been present in the common ancestor of all the Metazoan Triplo- 

 blastica." 



The point may be made here that by the indirect methods of renal 

 physiology it may be possible to prove filtration by the solenocytes with- 

 out an actual measurement of the pressure they are capable of exerting, 

 just as the vertebrate renal physiologist has measured the blood flow 

 through the kidney without operation, oncometer, or strohmuhr ! Further 

 work on these interesting forms is much needed. 



Reabsorption 

 It would appear that a major supporting argument for the process of 

 filtration is the presence of special mechanisms for reabsorption of ma- 

 terials from the kidney lumen. If urine were to be formed almost entirely 

 by the process of secretion, as appears to be true of the aglomerular fish, 

 it would be much more economical to secrete only those substances to be 

 rejected from the body along with a minimum amount of water for their 

 solution. A urine started as a filtrate, on the other hand, must contain the 

 electrolytes that were in solution, as well as glucose, and if the urine starts 

 as a coelomic fluid there may be considerable quantities of protein and 

 other large molecules in the fluid entering the kidney. For the conserva- 

 tion of these materials a reabsorptive process is needed as it would not 

 be in the secretory case. For an example, the aglomerular fish needs and 

 has no mechanism for the reabsorption of glucose. This argument cannot 

 be pushed too far, because the Malpighian body of the insect appears to 

 employ secretion on a large scale but with evidences of reabsorption, the 

 role of which is not yet understood. The evidences for reabsorption should 

 be examined at least for the groups for which an efifort has been made to 

 demonstrate the process of filtration. 



Molluscs 



The mechanism of filtration into the pericardial sac of fresh-water 

 lamellibranchs demonstrated by Picken, Florkin, and Potts would be 

 wasteful of the solutes of the blood. Picken (1937) believed that much 

 salt was reabsorbed as the filtrate passed through the kidney lumen. Pie 

 says : "The pericardial fluid is in most cases approximately isotonic with 

 the blood, but the urine is markedly hypotonic to the body fluid." 



Confining our attention to the "Cambridge water" specimens, the 



