FRESHWATER AND MARINE ORGANISMS. 331 



and water penetrate the tissues of marine animals faster than 

 they pass through the covering layers of freshwater animals. 

 Hut in both cases the presence of salts in abnormal proportions 

 in the medium renders the permeability of the integument high. 



The greater permeability to salts and water in marine organ- 

 isms accords with the ordinary conditions of existence for these 

 animals. Marine animals are constantly bathed in a medium 

 which is physiologically as suitable in inorganic composition a- 

 any internal one ( 1- redericq, '22). Freshwater animals, on the 

 other hand, cannot aflonl to lose dissolved substances from their 

 bodies nor to allow the entrance of water into their bodies up to 

 the point \\here o-motic equilibrium would result. 



< >-moti< pre--ure < hanges per se are evidently not particularly 

 deleterious to tin- \'t.tliiy of internal tissues, providing tin- in- 

 tegument does not attempt to regulate against them. In marine 

 organi-m- the freer penetration of solutes allows the internal 

 medium to keep pace with the outer medium as regards comp< si- 

 don, and so long as the salt balance i- prc-ervrd, no essential 

 turn I ion- are inhibited. 



Morphologista ha\c often attributed the chemical resistance of 

 hwater organisms to their {xjssession of an outer cuticulum. 

 Perhaps the resistance referred to is the resistance to the penetra- 

 tion of protein-precipitating agents, and in this a proteinaceon- 

 cuticulum probably asi-ts. But under ordinary conditions the 

 cutii ulum is by no mean- either impermeable or -i-mipermeable. 

 Rather the living integument is responsible for the maintenance 

 ol restricted or selecti\e permeability. 



III. 



A second significant characteristic of freshwater animaN ap- 

 pears to be that their body lluids have a lower osmotic pres-un- 

 than those of marine organisms. 



Whether there is a relation between toxicity and tonicity we 

 have attempted to investigate by measuring the surviv.il of 

 plasmoly/ed Spirogyra filaments. With a variety of plasmolys- 

 ing agents it was found that the toxic concentration was almost 

 exactly the lowest one which produced distinct permanent 

 plasmolysis. By gradually increasing the concentration of the. 

 medium, both toxirity and pl.i>molysis were prevented. Now, in 



