NITROGENOUS EXCRETION 65 



gocytes according to these authors, but this has been discounted by 

 Bahl (1947b), who also disposed of the idea that they are uric acid. 

 Guanine is not soluble in ammonia or acetic acid or dilute alkah, 

 but the granules of the nephridium are, and they also dissolve to 

 give a coloured solution in pyridine. This solution has absorption 

 bands at 558 m^u. (a) and 527 m/x (/S), and Bahl suggests that this is 

 a haemochromogen deriving as a breakdown product from the 

 blood. Further work may clarify this point since protoporphyrin 

 (Dhere, 1932; Laverack, 1960a) is known to be laid down in the 

 body wall during life, giving characteristic iridescence to the 

 integument, and this substance is soluble in pyridine. The 

 absorption peaks differ from those of the nephridial granules. 

 Roots (1960) has recently investigated the brownish-yellow pigment 

 of the chloragogen cells and found that this may be a complex 

 arrangement of substances which are soluble in ammonia and 

 pyridine, but possess no distinct absorption spectrum. This may 

 bear some resemblances to the nephridial material, but the question 

 needs further work. 



From his comparative studies on the blood, coelom and urine of 

 P.posthiima, Bahl (1947b) concludes that nephridia carry out three 

 functions in earthworm excretion, namely filtration, re-absorption 

 and chemical transformation. Filtration, in the sense of pouring a 

 liquid through a coarse sieve, but not ultrafiltration under pressure 

 through a membrane as in the vertebrate kidney, occurs at the 

 nephrostome. Thus in nephridia open to the coelom, protein 

 passes into the lumen of the nephridium, and it is unlikely that it 

 can do so from the blood vessels by ultrafiltration since the 

 pressure within these vessels is low {ca. 4-4 mm Hg at rest and 

 >9-3 mm Hg when active, Prosser et ai, 1950). As the protein 

 appearing in the urine is far less than that in the blood and coelomic 

 fluid, and much of what is present is accounted for by the secretions 

 of the body wall, it follows that much active re-absorption of 

 protein occurs through the nephridial wall against a concentration 

 gradient. Electrolytes such as chloride, sodium, potassium and 

 phosphate all diminish in quantity as they pass along the tubules 

 since the final concentration is lower in the urine than in the 

 coelomic fluid and blood. Although the evidence is not conclusive, 

 Ramsay's observation that urine becomes hypotonic in the wide 

 tube, and possibly more so in the middle tube is also suggestive of 



