OSMOTIC PRESSURE OF SEA WATER. 269 



the sea water, the deficit of salts in the blood of this latter group 

 is compensated by the osmotic pressure of the urea in the blood. 



3. The osmotic pressure of teleost blood is about half that of 

 the sea water (J =~o.S to -- 0.96 C). 



4. A dilution or concentration of the aquarium water always 

 causes an equivalent change in the blood of invertebrates, and 

 osmotic equilibrium between " internal and external media " is 

 established. Their membranes are completely permeable. This 

 permeability is proven for the integument of worms and the gills 

 of Limulus. 



5. Dilution or concentration of the aquarium water causes a 

 change in the same sense in the blood of selachians, but death 

 ensues before osmotic equilibrium is established. The mem- 

 branes of selachians are semi-permeable. 



6. Only slight if any change takes place when teleosts (Anguilla) 

 are transferred from salt to fresh water and vice versa. Normal 

 Fundulus heteroclitus will live in water varying in osmotic pres- 

 sure from that of the tap to sea water concentrated to double its 

 strength. The membranes of teleosts are impermeable, or the 

 fish possess some regulative mechanism which keeps the osmotic 

 pressure of the blood nearly constant. Extensive abrasion of 

 the skin of Fnndulus results in death in aquarium water of less 

 or greater osmotic pressure than that of their blood, for example 

 they die in fresh water and in normal sea water but not in sea 

 water diluted with an equal volume of distilled water. 



PHYSIOLOGICAL LABORATORY, 

 COOPER MEDICAL COLLEGE, SAN FRANCISCO. 



LITERATTRE. 



1. Bottazzi, F. 



'97 Archives ital. de biologic, 1897, XXVIII., p. 6l. 



2. Ph. Bottazzi and Enriquez. 



'01 Archiv. f. Anat. u. Physiol., 1901, p. 109. 



3. Fredericq, L. 



'82 Bull, de 1' Acad. roy. de Belgique, 1882. 



4. Fredericq, L. 



'98 Bull, de 1'Acad. roy. de Belgique, 1898, p. 831. 



5. Fredericq, L. 



Bull, de 1'Acad. roy. de Belgique, 3 ser, Tom. IV. 



