Water 



63 



osmotic stability. If ocean water becomes too dilute, as in the upper Baltic, 

 poikilosmotic animals disappear. The only ocean animals in which a sharp 

 gradient is maintained are marine vertebrates. Elasmobranchs, which retain 



TABLE 7 



urea, have a gradient in the same direction as in hesh water. In other marine 

 vertebrates some method of salt secretion exists and the blood is hypotonic. 

 Marine teleosts excrete salt against a gradient by their gills, marine mammals 

 by their kidneys. One important reason why insects and amphibians have 



