Swi 



mmtng 



249 



the accompanying figures. These changes when fol- 

 lowed thro the year show a rather distinct correspon- 

 dence to the seasonal changes in viscosity of the water. 



Fig. 152. Seasonal form changes of the water-flea, Bosmina coregoni. The 

 fractional figures above indicate date : those below indicate corresponding 

 temperatures in °C. (After Wesenberg-Lund.) 



Sivimming—F or rapid locomotion through the water 

 there are numberless devices for propulsion, but there 

 is only one thoroly successful form of body; and that 

 is the so-called "stream-line form" (fig. 

 153). It is the form of body of a fish: 

 an elongate tapering form, narrowed 

 toward either end, but sloping more 

 gently to the rear. It is also the form 

 of body of a bird encased in its feathers. 

 It is probably the form of body best 

 adapted for traversing any fluid medium 

 with a minimum expenditure of energy. 

 The accompanying diagram explains its 

 efficiency. The white arrow indicates 

 direction of movement. The gray lines 

 indicate the displacement and replace- 

 ment of the water. The black arrows 

 indicate the direction in which the 

 forces act. At the front the force of 

 the body is exerted against the water; 

 at the rear the force of the water is exerted against the 

 body. The water, being perfectly mobile, returns 



Fig. 15,-?. Stream- 

 line form. For 

 explanation see 

 text. 



