328 EDWARD F. ADJDLIMI. 



if the diluting fluid was distilled water as if it was a sugar solution 

 isotonic with sea water. 



It is important to realize that various groups of animals differ 

 greatly in their ability to control the interchange of substances 

 between body fluids and external environment (Semper, '80). 

 In all metazoa the covering epithelial layer of the body carries 

 on an active regulation of the interchanges, so that the outer 

 medium affects the more vital tissues only after it has with sur- 

 prising slowness changed the composition of the internal medium 

 (Adolph and Adolph, '25). Thus in all these animals there are 

 two lines of defense, two regulating surfaces which must be 

 penetrated. To illustrate several grades of this ability to control 

 the interchanges at the body surface, we may cite the mammals, 

 in which it is well known that the skin is impermeable at all 

 times; the aquatic amphibia, in which the permeability is con- 

 trolled in such a way that significant chemical interchanges 

 between body and environment are strongly opposed; and the 

 annelid worms, in which the interchanges are still less selected. 

 In the teleost fishes essential interchanges are limited to the gill 

 surfaces (Sumner, '06). In elasmobranch fishes the entire skin 

 is partially permeable, while in invertebrates the body fluid 

 often interchanges freely with the medium. 



We have attempted to compare the vital resistance to the 

 penetration of chemical agents in freshwater animals and marine 

 animals. For this purpose we have chosen those animals whose 

 entire body surface is normally permeable in some degree, 

 namely, small invertebrates. In them the surface mass ratio is 

 sufficiently large that effects of the chemical environment quickly 

 manifest themselves. 



II. 



An excellent measure of the amount of integumentary regula- 

 tion performed by organisms is the rate of change of body weight 

 when the organisms are transferred from their usual medium to 

 a solution of different chemical composition (Adolph and Adolph, 

 '25). Thus, when freshwater animals are immersed in salt solu- 

 tions there is a rather sudden loss of water, so that the- body 

 volume quickly reaches a new lower level. Seawater animals 

 placed in diluted sea water gain in volume, even though the 



