248 Adjustment to Conditions of Aquatic Life 



of the adjustment that keeps a whole population in the 

 water afloat near to, but not at the surface. This comes 

 out most clearly, perhaps, in those minor changes of 

 form that accompany seasonal changes in temperature 

 of the water. In summer when the viscosity of the 

 water grows less (and when in consequence its resist- 



a 



b 



PIG. 151. Summer and winter forms of plancton animals : sum- 

 mer above, winter below, a, the flagellate Ceratium; b, the 

 rotifer Asplanchna; c, d, e, water- fleas; c and d, Daphne; 

 e, Bosmina. (After Wesenberg-Lund). 



ance to sinking is diminished) the surface of many 

 plancton organisms is increased to correspond. The 

 slender diatoms grow longer and slenderer, the spines 

 on certain loricate rotifers grow longer. Bristles and 

 hairs extend and plumes and fringes grow denser. Even 

 the form of the body is altered to increase surface- 

 contact with the water. A few examples are shown in 



