164 FOUNDERS OF OCEANOGRAPHY 



with some power of locomotion (such as Sagitta) descend to 

 a moderate depth, under twilight conditions, during bright 

 dayHght, and come to the surface at night. Michael, and also 

 Esterley and others working on the Californian coast, have 

 demonstrated this diurnal migration in relation to light for 

 many of the larger and more active members of the plankton, 

 and the general principle of avoiding bright sunlight prob- 

 ably holds true for most, if not all, of the zoo-plankton. The 

 largest catches of plankton are obtained, in most seas, not 

 on the surface, but at a depth of 5 to 10 fathoms. Moreover, 

 some of the bottom-living animals, such as Amphipods, 

 Cumacea, and other higher Crustacea, are known to come to 

 the surface at night. 



Many of the " bathypelagic " animals which remain below 

 the photic zone show peculiar adaptations to the absence of 

 sunlight, such as the characteristic red colour, modification 

 or loss of eyes, presence of special light -producing organs, and 

 the development of tactile appendages. 



Viscosity. 



The viscosity of sea-water — the resistance it offers to a 

 body falling through it — varies greatly with the temperature, 

 and is much greater in cold than in warm water. Conse- 

 quently, in polar seas, where the viscosity is great, there is 

 little or no change in the amount in passing from the surface 

 to the bottom, while in the tropics the small degree of vis- 

 cosity in the warm surface water rapidly increases in passing 

 to deeper and colder layers — as the temperature falls the 

 viscosity increases. 



Taking the value of the viscosity of pure water at freezing- 

 point as being 100, then in sea- water having a salinity of 

 35%o (per'mille) the viscosity at a few temperatures such as 

 would be met with in the tropics between surface water of 

 over 80° F. and the cold bottom water would increase, as 

 shown in this table (adapted from Murray) : — 



