EXCRETION 



297 



physiology of nephridia in lower animals, and any role that they may have 

 in voiding nitrogenous excreta still awaits quantitative analysis. Other 

 suggested functions are osmoregulation and ionic regulation, and in the 

 earthworm it has been discovered that a urine is secreted which is strongly 

 hypotonic to the body fluids (42). 



In marine gastropods and lamellibranchs ammonia is an important 

 constituent of nitrogen excretion, but various species, notably Mytilus, 

 have an appreciable amino-acid fraction (Table 7.2). No urine analyses are 



Vent rat 

 lip 



Nephridiopore 



Body wall 



Fig. 7.1. Excretory Ducts of Worms 



(a) Typical solenocyte of a polychaete; (b) inner end of a nephridial canal of a polyclad 

 turbellarian, with flame cell at extremity; (c) metanephromixium of Odontosyllis. 

 Dorsal and ventral lips pertain to the nephridiostome. (From Goodrich.) 



available but it seems likely that some fraction of the total excretory-N is 

 discharged in the urine. There is evidence that fluid is filtered across the 

 heart wall into the pericardium and then enters the kidneys. By producing 

 a urine hypotonic to the blood the kidneys of freshwater species participate 

 in the maintenance of a hyperosmotic internal milieu. 



The total output of non-protein nitrogen in a medium-sized octopus 

 amounts to about 25 mg/day. Half of this is ammonia, but there are ap- 

 preciable amounts of urea and amines, amounting to 15 and 20% res- 

 pectively. Since the data are not related to body weight, it is impossible 

 to partition nitrogen excretion by renal and extrarenal routes. The urine 



M.A. — 10* 



