THE SEA 25 



depths does not crush them, for their bodies are in 

 many cases thoroughly permeable to water. Many 

 deep-sea animals are totally blind, while many others 

 are phosphorescent. 



Deep-sea animals are much more delicately con- 

 structed than their shallow-water relatives. The 

 crabs have very spindling legs and frail bodies. The 

 fishes have very soft and velvety skin, with either a 

 very few scales or none at all. The shells of the 

 moUusks are very thin and brittle. 



One phase of aquatic life, seldom considered by 

 most persons, is of great economic importance. Vast 

 numbers of minute organisms float or drift in the 

 sea with little or no power of independent locomotion, 

 and are known collectively as plankton. The organ- 

 isms composing plankton are chiefly minute algae 

 (diatoms, desmids, and filamentous algae), protozoa 

 (flagellates and rhizopods), rotifers, and minute 

 Crustacea (cladocera and copepoda). These minute 

 organisms constitute the chief food of many species 

 of fish, and indirectly constitute the bulk of the food 

 of all fish, for those which do not feed on plankton 

 live on fish or other organisms which subsist on 

 these small plants and animals. Plankton is most 

 plentiful near the mouths of large rivers, as it 

 depends for its food to a large extent upon the 

 combined nitrogen brought in by the rivers. 



The ocean is so vast, its conditions are so varied, 

 and its forms of life so multitudinous in number that 

 it is impossible to give more than a brief survey of 

 the interesting facts about it in this chapter. 



