64 THE PHYSIOLOGY OF EARTHWORMS 



Structures are available from Bahl (1947), Goodrich (1945) and 

 Stephenson (1930). See also the section on water relations and the 

 work of Ramsay (1947, 1949). 



Basically the nephridium is a tube, often open at both ends, 

 internally and externally, and lined with cilia in tracts along its 

 length. There is a good blood supply to this organ. The tube is 

 specialized into distinct regions and the function of these regions is 

 only partially known. 



The many ciliary tracts that line the tubular nephridium are 

 continually beating such that fluid passes along the lumen from the 

 internal end to the external end. In those nephridia that are closed 

 internally the tubular contents must obviously arise by filtration 

 through the walls. In species having nephridia opening internally 

 the contents may arise equally or unequally from the coelom and 

 blood supply. If the majority of the fluid comes from the coelom 

 then the chemical composition must be changed considerably 

 during its passage along the tube. If it originates from the blood 

 then many of the larger molecular substances such as protein, 

 amines and organic food substances may be precluded from 

 passing through the nephridial wall and thus never appear in the 

 urine at all. Electrolytes pass through the walls into the lumen in 

 both situations. 



It is known that the composition of the urine changes as the 

 fluid passes along the nephridium, Ramsay (1949) showing that the 

 osmotic pressure changes are mainly a function of the ''wide 

 tube" (see Chapter VI). No work has been published to show 

 whether salt resorption or water secretion occurs at this stage, 

 although the phenomenon of granular resorption has been 

 described by Cordier (1934). He found that small granules, less 

 than lOOOA can be removed directly to the exterior via the 

 nephridium. This compares with an upper size limit of 25A 

 passed by vertebrate kidney nephrons. Some granules in the 

 earthworm, however, are accumulated by the ciliated middle tube 

 wall, and the granules thus taken up are thought to be permanent 

 fixtures in this situation until the end of life (Stephenson, 1930; 

 Bahl, 1947b). The middle tube of the nephridium thus seems to 

 act as a kidney of accumulation. It was suggested by Willem and 

 Minne (1900) that the naturally occurring granules observed in 

 this situation are guanine, in common with the granules of chlora- 



