118 



How Animals Changed 



(Schlieper, 1929, 1933) . Crustaceans, in order to attain equilibrium 

 with the surrounding medium in dilute solutions, excrete water 

 largely through the kidneys (Schlieper, 1933), but fishes lose water 

 chiefly through their gills and other external membranes (Dakin, 

 1912). 



A fresh-water clam has little ability to resist osmotic changes in 

 hypertonic solutions, and dies rather quickly in salt water (Fig. 19) , 

 but a crayfish is more hardy. The former has little salt and little 

 organic matter in its blood; the latter has much of both. A crayfish 



CARCINUS 



TELPHUSA 



ANGUILLA 



CYPRINUS 



ANODONTA 



2° 



A,EXTERNAL MEDIUM 



Fig. 19. The relation between molecular concentration of the blood 

 and that of the external medium in various aquatic animals: Ano- 

 donta sp., Carcinus maenas, Telphusa fluviatile, Anguilla, and 

 Cyprinus carpio. (After Schlieper, 1933.) 



