OSMOTIC AND IONIC REGULATION 233 



sulphate may penetrate in accordance with the diffusion gradient in those 

 animals in which the plasma levels of these ions are low. 



I have recently made careful analyses of sea water in which Carcinus 

 nmenas has been kept in order to obtain the net uptake or output of water 

 and ions. In this crab the secretion from the antennal glands amounting to 

 about 5 % of the body weight per day is balanced by the absorption of an 



TABLE 3. IONIC BALANCE IN INTERMOLT PHASE OF CRABS 

 (CARCINUS MAENAS) 



The net uptake or output (actual) and the output of urine (antennal gland secretion) 

 over two 7-day periods are given for two crabs. Crabs were kept individually in 500 

 ml. sea water containing in mg. Na 5022, K 182, Ca 192, Mg 606, CI 9050, SO^ 1263. 

 Uptake or output is not significant [ ] unless it exceeds the following values : Na 

 24.1, K 3.8, Ca 2.1, Mg 7.5, CI 11.5, SO^ 15.2. 



* Calculated figures based on average composition of this secretion, and average 

 uptake of fluid, 4.7% of body weight per 24 hr. (Webb, 1940) . 



equivalent volume of fluid through the gills (Webb, 1940). The results 

 of balance experiments in two crabs are given in Table 3. With respect to 

 sodium, chloride, and magnesium, the crabs are essentially in balance 

 within the accuracy of the analytical methods, the output through the 

 antennal glands and other possible sources such as the gut being offset by 

 uptake of similar quantities of ions, the sodium and chloride against a 

 concentration gradient. Potassium balance over a weekly period is nega- 

 tive, and most of the loss would seem to be in the secretion from the glands. 

 Calcium is also lost from the crabs in small quantities, which are usually 

 less than the amounts calculated as being excreted, so a slight uptake may 

 have taken place. Results with sulphate show a net loss of this anion, 

 but from the amounts excreted by the antennal glands it is clear that con- 

 siderable uptake may take place, amounting to more than half of that ex- 

 creted. 



It seems, therefore, that the incoming fluid contains all ions except 

 perhaps potassium, these ions with the exception of magnesium and sul- 

 phate being taken up against concentration gradients. 



