68 THE PHYSIOLOGY OF EARTHWORMS 



content of the final fluid can be expected to be less than that of the 

 coelomic fluid. 



Summary 



Nitrogen is lost in the form of mucus secreted by the epidermis 

 and in urine. 



Earthworms excrete ammonia, urea and also, possibly, uric 

 acid, allantoin and doubtfully creatinine. The concentrations of 

 these substances varies according to the diet. In animals under 

 normal conditions of food and water intake, ammonia is the 

 dominant constituent in urine with smaller quantities of urea; 

 under conditions of starvation the position is reversed, urea 

 increasing above the previous level and above that of ammonia. 

 Some evidence is available that the chloragogen cell is involved in 

 excretion probably via the Krebs— Henseleit cycle of reactions. The 

 enzyme arginase, and the amino-acids ornithine, glutamic acid and 

 arginine have been shown to be present but not citrulline. Hetero- 

 xanthine may be present in chloragogen. There are indications that 

 re-absorption of material occurs in the nephridia but filtration from 

 the blood and coelom may be efficient enough to account for 

 differences in analyses of urine, blood and coelomic fluid. 



