Body Fluids 117 



cyanide. The parenchyma soon returns to its original condition and 

 resumes its normal activity. Then the animal sets up a resistance, 

 and the integument becomes less permeable, but the gut remains 

 vacuolated as long as it is in dilute water. The excretory system 

 does not appear to be concerned particularly with such regulatory 

 processes. "Distinction must be drawn between the ultimate imper- 

 meability of the ectoderm considered as a membrane and the os- 

 motic resistance of the individual cells of this layer and of other 

 tissues. The permeability of the ectodermal membrane can be re- 

 versibly increased by calcium deficiency, but the osmotic resistance 

 of its individual cells cannot be broken down by this means." Both 

 the vacuole formation by the gut and the osmotic resistance devel- 

 oped by individual cells are believed to be active processes which 

 expend energy, but the impermeability of the external membrane is 

 a passive process. 



The salinity of body fluids of animals is influenced chiefly by 

 three factors: (1) external membranes, (2) products of metabolism 

 present in the internal medium, and (3) renal organs (Dakin, 

 1912) . In fresh water the blood of crustaceans generally has a 

 higher salinity than that of moUusks. Such differences are inherent 

 and dependent on characteristic racial adaptations. In the ocean the 

 salt content of the bloods of sharks and bony fishes is similar. But 

 the blood of a shark is isotonic with sea water, its osmotic pressure 

 being maintained by the presence of large quantities of urea, while 

 that of a teleost has only about a third of the osmotic pressure of 

 sea water. European littoral crabs present a graded series in regard 

 to their ability to maintain a constant internal medium: the blood of 

 Hyas rapidly becomes isotonic with sea water or dilutions of it 

 which surround its body; Cancer changes much more slowly; and 

 Carcinus is little influenced by changes in the salinity of the water 

 around it. When placed in diluted sea water Hyas absorbs water 

 and increases in weight rapidly; Cancer changes more slowly, and 

 some individuals change very little; Carcinus does not change 



