250 INVERTEBRATE PHYSIOLOGY 



such a way as to contribute to an understanding of mechanism. The final 

 product will lack the grace of form which has been given to the same story 

 told phylogenetically by Carl Schlieper in the Fortschritte der Zoologie, 

 Vol. 9 (1952), and by G. S. Carter in A General Zoology of the Inverte- 

 brates (1951). The reader is referred to those works for considerable 

 detail that must be omitted here. The result of this approach, it is hoped, 

 will be critical comparison of the part played by these mechanisms in urine 

 formation. 



Filtration 



It is most profitable to begin our discussion with a consideration of 

 filtration ; for where filtration of the blood occurs in the process of urine 

 formation it will be an initial process, producing a fluid which will probably 

 be modified by other processes before the discharge of the urine from the 

 body. When these other processes are inhibited, the composition of the 

 urine will revert more and more nearly to that of a filtrate and in this rela- 

 tionship we have ready at hand a test for the presence of filtration. Of the 

 invertebrate animals as a whole it might be well to choose to start with a 

 phylum for which a considerable body of evidence has been accumulated 

 and to pass from well-established facts to a consideration of the groups for 

 which only inferences can be made. Three different molluscan classes have 

 yielded evidence of one kind or another for filtration. 



Molluscs 



In a general way it would appear essential that fluid accumulating in a 

 coelomic cavity be in some sort of steady-state relationship to blood, and 

 that filtration through the lining of the coelom would participate to a 

 greater or lesser degree in maintaining this steady state. Whenever we deal 

 with a coelomic fluid, consequently, we are already dealing in part with a 

 filtrate. With the elaboration of a part of the coelom for excretory 

 functions, this primitive filtration might be refined or suppressed. Molluscs 

 possess a reduced coelom of which one part becomes a pericardial chamber, 

 another the cavity of the renal structures (Goodrich, 1945). These cham- 

 bers commonly remain in contact one with the other through a renoperi- 

 cardial canal and may participate together in the excretory function. The 

 renopericardial canal is frequently ciliated, the beat being in the direction 

 of the kidney lumen. Movement of fluid through the lumen of the glandular 

 portion of the kidney may then be accomplished either by the contraction 

 of muscles within the kidney walls or by pressure set up by general body 

 movements. 



In the lamellibranch molluscs the atria are very thin-walled structures 

 in which the pressure is rhythmically elevated above the general tissue 



