3] ACCLIMATIZATION" 85 



3. ACCLIMATIZATION TO SOLUTIONS OF GREATER OR LESS 

 DENSITY THAN THE NORMAL 



In the preceding section we saw that different organisms had 

 a diverse resistance period to the same density of solution. In 

 part, this may be accounted for, as we have seen, on the ground 

 of a difference in the rapidity of osmotic action thick-skinned 

 animals resisting longer than thin-skinned ones. All diversity 

 in the effect of solutions, cannot, however, be accounted for on 

 this ground. Thus, the molluscs of the sea and those of fresh 

 water appear to have an equally pervious epidermis, yet the 

 former will, of course, withstand a much stronger solution of 

 salt than the latter. This difference in resistance capacity 

 seems closely correlated with the conditions of the medium in 

 which the organism has been reared. Thus, BEUDANT ('16) 

 found that littoral species (living, therefore, in a part of the sea 

 where the water is much diluted by rivers), e.g. Ostrea, Mytilus, 

 Patella vulgata, resist fresh water better than deep-sea species; 

 and this discovery has been abundantly confirmed by DE 

 VARIGNY ('88).* ' 



That the conditions of density of the culture medium deter- 

 mine the resistance capacity is proven by experiment, for, by 

 varying the density of the culture solution, we may vary the 

 resistance period of the individuals experimented on. BEUDANT 

 ('16) was the first to show this. He used Lymnea, Physa, 

 Planorbis, Ancylus, Paludina, and some other fresh-water Mol- 

 lusca. He began in April by putting these organisms into a 

 1% NaCl solution, and, continuing to add salt slowly, by Sep- 

 tember many of these withstood a 4% solution a solution 

 which kills animals suddenly subjected to it. He performed 

 likewise the reverse experiment upon marine Mollusca (Patella, 



* The extremes of density in which organisms are capable of living are often 

 considerable. On the one hand, the individuals of some species, especially fish, 

 are able to migrate from fresh to salt water and back, with impunity. On the 

 other hand, many species of a family, the other members of which are marine, 

 have become accustomed to fresh water. Examples of this last case are the 

 hydroid Cordylophora lacustris, the mollusc Dreissena, and the endoproctan 

 bryozoan Urnatella. Likewise, some marine species have come to live in exces- 

 sively salt water. Such, for example, is the case with Artemia salina which 

 lives in Salt Lake, Utah, containing over 22% of salts. (LEIDY, '72, p. 165.) 



