1916 



Shelford; on Physiological Differences 



167 



Table 5. Shoxving the resistance of crabs of the same and. different species 

 from different depths, to fresh water. 



♦From Station D. 

 **From Station B. 



it will be noted that the crabs from deeper water are less resistant. The 

 purple shore crab is most resistant and lives out of water at low tide. 



Table 6 shows the other animals on which similar experiments were 

 performed. Here again the methods employed with the crabs <■■ e used. 

 Animals belonging to the same group showed the same general relations 

 as in the other case. The animals such as barnacles and mussels which 

 close the shell in fresh water live for long periods. The time given is 

 the time which the animals were actually in fresh water, as it was neces- 

 sary for them to be placed in the salt water for a time to determine their 

 conditions. 



C. Resistance to Acidity and Alkalinity 



By acidity we mean that the concentration of hydrogen ions is in 

 excess of the concentration of hydroxyl ions ; by alkalinity we mean the 

 excess of hydroxyl ions over hydrogen ions. Moore and Whitley (It) 

 found that an addition of .0015 mol of caustic alkali or .001 mol of acid 



