278 THE EFFECT OF MOISTURE 



bons * for certain tropical and sub-tropical species of 

 Littorina. These organisms are confined to more or 

 less brackish waters, and seem incapable of living in 

 pure salt water. Gibbons says he has " met with three 

 of these species, and in each case they have been dis- 

 tinguished from the truly marine species by the ex- 

 treme (comparative) thinness of their shells, and by 

 their colouring being richer and more varied; they are 

 also usually more elaborately marked." Thus diminu- 

 tion of salinity seemed to have produced thinness of 

 shell in the species as a whole, but within their own 

 limits it was found that the reverse relation held, and 

 that, as in Bateson's observations, the shells became 

 thinner as the water they lived in became more salt. 



In marine animals, as in fresh-water ones, increase 

 of salinity probably tends to diminish the rate of 

 growth. Decrease of salinity, on the other hand, may 

 have the reverse effect, and within certain limits actu- 

 ally increase the growth rate. Thus Loeb t determined 

 the rate of regeneration of decapitated hydroid polyps 

 (Tubularia mesembryanthemum) placed in sea-water of 

 various degrees of dilution and concentration, seven to 

 nine individuals being measured at each concentration. 

 His results are reproduced in the figure given below. 

 Here the Specific Gravity of the water is represented 

 along the abscissa, and the amount of regeneration by 

 the height of the ordinates. We see that the maximum 

 rate of regeneration took place in water of Sp. G. 

 1.025, or in very considerably diluted sea-water (the Sp. 

 G. of this being about 1.038). At this dilution the re- 



* Quart. Journ. Conch., i. p. 339. 



t " Biological Lectures delivered at Wood's Holl," 1893, p. 46. 



