140 PHYSIOLOGY OF THE INVERTEBRATA. 



The blood of crabs living in brackish water contains a 

 smaller percentage of saline matter than those living in sea 

 water ; and the blood of crayfishes living in rivers contains 

 only a very small amount of saline matter — generally less 

 than one per cent. 



According to the above investigations it appears that there 

 is an exchange of salts, forming a kind of equilibrium 

 between the composition of the blood and the external 

 medium in which these Crustaceans live. This equilibrium 

 is the result of the simple laws of diffusion. 



Among the fresh water Crustaceans the albuminoid 

 substances of the blood probably retain a little more 

 of the soluble salts than is contained in the external 

 medium. 



It is probable that this exchange of dissolved salts is 

 established by the respiratory organs — the branchiae. The 

 delicate walls of the branchias, which separate the blood from 

 the external medium, allow the respiratory gases to pass by 

 simple diffusion : and most likely these delicate walls act .in 

 a similar manner to a dialyzer with easily diffusible salts. 

 The albuminoid substances of the blood do not pass into the 

 external medium. 



The nourishing fluids, to which the illustrious physiologist 

 —Claude Bernard — gave the name of " milieu intdrieur,'' 

 have not (with the animals previously mentioned) the 

 constant chemical composition and independence of the 

 conditions of the " milieu ext6rieur " which characterises the 

 blood of the higher animals. 



Among fishes (Pisces) the branchial walls allow equally to 

 pass the oxygen and carbonic anhydride of respiration. One 

 can therefore understand that there is a similar exchange of 

 salts between the blood and the external medium. But 

 experience proves that it is the inverse of that which takes 

 place among the Crustaceans and other Invertebrates ; for 

 the blood of marine fishes has a saline composition which is 

 entirely different from that of sea water. The blood of a sole, a 



