DRIFTING LIFE 129 



down movements are the changes in the strength of light 

 experienced by the animals ; this is further confirmed by the 

 fact that many animals live deeper in the bright sunny days 

 of midsummer than they do earlier in the year, when the 

 light entering the water is not so strong. From this, then, 

 we see that most of the plankton animals tend to avoid 

 strong light and prefer the dimly lit conditions of the deeper 

 layers ; at the same time all the evidence goes to show that 

 each animal shows a preference for a certain strength of 

 light to which it is adapted. Towards the evening they 

 follow the " optimum " strength of light towards the 

 surface as night draws on, but in the dark there is no light 

 stimulus and they are free to move anywhere (see Plate 48) ; 

 at dawn they once more pick up their optimum intensity 

 and move downwards as the daylight strengthens. It is also 

 a curious fact that the older an animal becomes the more it 

 shuns the light, and it is generally the younger stages that 

 are found near the surface. This is not, however, always 

 the case ; the early stages of the Velella or " By the wind 

 sailor " are found at very deep levels, while the adult, as 

 mentioned below, floats right on the surface of the water, 

 where it is blown hither and thither by the winds. 



Adaptations for Suspension 



In considering this drifting life it may have struck the 

 reader that it is a curious thing that all these organisms, 

 many of which, such as the diatoms, are practically in- 

 capable of any independent movements, should remain 

 suspended in the water. Why do they not sink rapidly 

 to the bottom of the sea ? The answer to this question 

 rests in the demonstration of some of the most remarkable 

 structural adaptations, which fit most of the plankton 

 organisms to the conditions under which they live. 



Their main requirement is that by some means or other 



