244 Adjustment to Conditions of Aquatic Life 



ity in sinking is determined by several factors, one of 

 which is external and the others are internal: 



The external factor is the varying viscosity of the 

 water. 



The internal factors are specific gravity, form and 

 size. 



We have mentioned (p. 30) that the viscosity of the 

 water is twice as great at the freezing point as at 

 ordinary summer temperatures ; which means, of course, 

 that the water itself wotild offer much greater resistance 

 to the sinking of a body immersed in it. We are here 

 concerned with the internal factors. 



Lessening of specific gravity — The bodies of organisms 

 are not composed of living substance alone, but con- 

 tain besides, inclusions and metabolic products of 

 various sorts, which oftentimes alter their specific 

 gravity. The shells and bone and other hard parts of 

 animals are usually heavier than protoplasm; the fats 

 and gelatinous products and gases are lighter. We 

 know that the fats of vertebrates, if isolated and thrown 

 upon the water, will float; and that a fat man, in order 

 to maintain himself above the water, needs put forth 

 less effort than a lean one. There are probably many 

 products of the living body that are retained within 

 or about it and that lessen its specific gravity, but the 

 commonest and most important of these seem to fall 

 into three groups: 



1. Fats 077 d oils, which are stored assimilation 

 products. These are very easily seen in such plancton 

 organisms as Cyclops (see fig. 96 on p. 189) where they 

 show through the transparent shell as shining yellowish 

 oil droplets. Most plancton algse store their reserve 

 food products as oils rather than as starches. 



2. Gases, which are by-products of assimilation, and 

 are distributed in bubbles scattered through the tissue 



