76 



THE PHYSIOLOGY OF EARTHWORMS 



pores is always hypotonic to the body fluids except in the most con- 

 centrated media (> 1-0% NaCl) (Table 10). It is thus clear that the 

 earthworm has no powers of osmoregulation in concentrated media, 

 although the chloride data alone may suggest such a course. In fact 

 survival in very concentrated media is often not for very long, and 

 urine production is very limited. 



It is evident then that the earthworm, so far as is known at 

 present, functions like a fresh-water animal, albeit with some 

 anomalies. The maintenance of an internal O.P. above that of the 

 environment, save in concentrated media, the elimination of 

 hypotonic urine, the absorption of salts from very dilute solutions, 



Table 10 



Total Osmotic Pressure and Proportion Due to Chloride as 

 % NaCl in Relation to the Concentration of the External 



and the volume of urine production (estimated at 60% of the 

 body weight in 24 hours by Wolf (1940)), all point towards a 

 fresh-water type of excretory pattern. The fact that the chloride 

 concentration is kept below that of the external medium does not 

 require the postulation of an active process of salt removal against 

 a concentration gradient. In media over 0-35% NaCl salt doubtless 

 diffuses in, possibly coupled with the active uptake process in- 

 volved in dilute solutions. But water also tends to enter since the 

 internal O.P. is higher than the external, and as this latter process 

 probably occurs more swiftly than the former, due to the greater 

 permeability of cells to water, the effect is that of dilution of the 

 coelomic fluid. The animal can thus maintain the chloride lower 



