196 THM INFLUENCE OF INANIMATE SUREOUNDINGS. 



river cray-fish. are kept in stagnant water, they soon die for 

 want of oxygen, or else they quit it and prefer to live in the air ; 

 it is well known that they are packed for carriage in wet moss, 

 and not in water. Further back I have already mentioned 

 (see p. 172) that fishes are easily drowned if they are prevented 

 supplementing the amount of air they derive from the water, 

 which is insufficient for their respiration, by swallowing 

 air at the surface. Fritz Miiller has made us acquainted 



Fig. 57.— a, Zvpea^ a swimming crab that breathes only in water ; 6, Oct/podaj a marine 

 crab which easily suffocates in water. 



with a few other examples. He showed that the Ocypoda, 

 which lives half its time on land and in part breathes air 

 (fig. 57, h), can easily be drowned if it is held in sea-water, 

 which yet contains enough oxygen to allow a Lupea diacantha 

 to recover itself perfectly when it has been almost killed by 

 being kept in the air. It follows from this that the osmotic 

 power of the respiratory dermal surface is extremely different in 

 the two animals, and that in the Ocypoda it is not great enough 

 to extract the considerable amount of Oxygen necessary to the 



