2] 



EFFECT ON STRUCTURE AND FUNCTIONS 



81 



them to solutions both more and less dense than the normal, 

 and determined their resistance periods. As a result of sub- 

 jecting fresh- water animals to salt solutions, he found that 

 their resistance period diminished approximately as the thick- 

 ness of the skin {and cuticula) diminished. Thus, when plunged 

 into sea water (3.046% salts) adult water insects resisted in- 

 definitely ; insect larvae 6 to 4 hours ; Entomostraca less than 

 an hour ; Nephelis, 5 to 7 minutes ; Planaria, 4 minutes ; and 

 Hydra only 1 minute. GOGORZA ('91) got similar results, find- 

 ing the resistance capacity of the different groups to diminish in 

 this order : molluscs, crustaceans, fish, worms, tunicates, echino- 

 derms, coelenterates. So we may consider this relation between 

 resistance period and thickness of covering a general law of 

 resistance ; and it is what we should expect upon the theory 

 that the solutions act osmotically. 



By subjecting organisms to separate solutions, each contain- 

 ing 3% of the various salts found in sea water, PLATEAU was 

 able to show that NaCl produced the most important effect, 

 MgCl 2 the next most important effect, and MgSO 4 still less. 

 This is shown by the following - 



TABLE IX 



RESISTANCE PERIODS OF FRESH-WATER CRUSTACEA TO VARIOUS CONSTITUENTS 

 OF SEA SALT. TEMPERATURE NOT GIVEN 



(Numbers indicate minutes elapsing before death occurred) 



Although from this table it seems clear that there is an 

 inverse relation between resistance period and osmotic index, 

 PLATEAU did not believe that the death of the animals experi- 



