PELAGIC ZONE— SURFACE LIFE 



449 



Pelagic Plankton. — The floating and drifting population of 

 the sea possess a number of common characteristics related to 

 their mode of life. They are typically translucent or transparent, 

 a feature due to the large proportion of water in their tissues. 

 By making some of them more or less difficult to see, this may 

 serve to some extent as a means of protection (see vol. ii, p. 278), 

 and by reducing the density of their bodies it must certainly 



Fig. 1304.— A Ray- Animalcule (Thalassicola pelagka) with "bubbly" protoplasm, much enlarged. 



render floating a comparatively easy matter. The latter purpose 

 is also promoted by arrangements of other kind. In many of the 

 minute crustaceans and crustacean larvs, for example, there are 

 numerous spines and hairs which must reduce the tendency to 

 sink. Oil -globules are of common occurrence, both in adult 

 animals and in some floating eggs, such as those of fishes. And 

 there may also be gas-receptacles for buoying up the body. In 

 some of the Animalcules, for example, the living substance 

 (protoplasm) of the animal is of "bubbly" consistency, owing to 

 the presence of minute spaces filled with liquid, or even gas 

 (fig. 1304). In many of the Compound Jelly-Fishes {Siphono- 

 phora) there is a gas-filled float at the upper end of the colony. 



