OSMOTIC AND IONIC REGULATION 239 



From the data available concerning osmoregulation in fresh-water ani- 

 mals, it is apparent that the mechanisms of the regulation vary in different 

 animals or groups. Lamellibranchs like Anodonta with large surfaces in 

 contact with the external medium and low metabolism are unable to main- 

 tain a high osmotic gradient between blood and water. Despite the low 

 gradient, the water flux of these animals, as measured by Potts (1945b) 

 from the rate of urine production, varies from about 5^ of the weight per 

 day at 0° C to 24% at 18° C. If the animal is narcotized with ether or 

 barbiturates, the production of urine stops, but the animal increases in 

 weight, owing to the osmotic uptake of water (Florkin and Duchateau, 

 1948). In an Australian fresh-water mussel Hyridclla australis, closure 

 of the shell forms an effective seal from the external medium, eliminating 

 weight changes in short-term experiments in different media, and allow- 

 ing the lamellibranch to survive periods of desiccation up to three months 

 (Hiscock, 1953). 



In the larvae of the mosquito Acdcs aegypti, the body wall is relatively 

 impermeable to water except across the anal gills or papillae, which also 

 actively absorb chloride from the environment. Using ingenious technical 

 methods for the small quantities of fluid available, Ramsay (1950, 1951, 

 1953) has shown that the fluid eliminated by the rectum is markedly hypo- 

 osmotic to the haemolymph, and contains less sodium and more potassium. 

 These ions enter the body through the anal papillae and are excreted by 

 the Malpighian tubules into the intestine in amounts which vary directly 

 with those in the external medium, but the sodium of this intestinal fluid 

 is always less and the potassium more than the values of these ions in the 

 blood. 



Sialis larvae, however, have no special ion-absorbing organs, and their 

 tracheal gills are in fact, as well as in name, respiratory organs. The excre- 

 tory fluid in the rectum, presumably produced by the Malpighian tubules, 

 is not strongly hypo-osmotic, since its conductivity is as much as 6Sfo 

 that of the blood ( Shaw, 1955a) . It contains large quantities of ammonium 

 and bicarbonate ions, is chloride-free, and has only small quantities of 

 sodium and potassium. Sodium, potassium, and chloride of the blood in- 

 crease slowly when the larvae are kept in dilute solutions of sodium or 

 potassium chlorides. At the same time the concentrations of these ions in 

 the rectal fluid increase, but only potassium exceeds the value in the 

 blood, reaching a concentration of three to six times. 



In these experiments most of the uptake of ions in Sialis takes place 

 from fluid absorbed into the gut. The osmotic intake of water through the 

 cuticle is much less than in Anodonta, about 4^c of the weight per day at 

 20° C. 



The gut of aquatic animals is much more permeable to water and ions 



