450 DISTRIBUTION IN SPACE AND TIME 



and sometimes (Velella, fig. 1305) a crest projecting from this 

 may almost be said to serve as a sail. 



It must not be supposed, however, that plankton animals are 

 always found at the surface, for, on the contrary, they are able 

 to withdraw themselves from it to a greater or less depth, and 

 thus avoid the damaging effects of a rough sea or an excess of 

 temperature. Our ignorance is at present too great to enable 

 us to explain the reasons for all the upward or downward move- 

 ments which constantly go on, sometinfes in a curious periodic 

 manner. As Hickson says (in The Story of Life in the Seas, 



Fig. 1305. Velella 



a little book which is heartily commended to the attention of 

 readers): " The fact is, that the conditions of life in the surface 

 waters are so complicated that it is extremely difficult for us to 

 accurately estimate the balance of the forces which act upon these 

 organisms. The direct heat of the sun, the light of both the sun 

 and the moon, the tranquillity or roughness of the sea, the con- 

 ditions of the tides and winds which cause changes in the surface 

 temperature of the water, independently of the direct heat of the 

 sun, all influence the delicate tissues of which these animals' 

 bodies are composed, and cause them to change their position." 

 Phosphorescence is another common property of plankton animals, 

 and its meaning is in many cases difficult to understand. Plank- 

 tons are of very various character. Some contain animals of 

 many different species, others consist of a single form of life. 



Vertebrates (Vertebrata) of the Plankton. Among Fishes 

 (Pisces) occasion has already been taken to note (see vol. iii, 



