ADAPTATION 215 



Plankton. In contrast to benthonic animals are those which 

 merely float in the water, drifting about with its movements. This 

 group is called plankton, and comprises innumerable species both 

 of animals and plants. Single-celled organisms are very numerous. 

 The Coelenterata are also well represented, and molluscs of some 

 classes. The plankton includes the most extreme aquatic organ- 

 isms, such as the jelly-fishes. Radial symmetry is common in this 

 group as in the sedentary benthos, and for the same reason, lack 

 of locomotion. Transparency is also common in the plankton. 



Nekton. The transition from plankton to the third division of 

 aquatic life, the nekton, is gradual. The essential characters of 

 organisms belonging to the nekton are correlated with their ability 

 to move freely through the water, resisting all motion of the me- 

 dium in which they live. This necessitates a non-resistant body 

 form, well illustrated by the spindle-shaped bodies of common 



Fig. 126. — The herring, Clupea harengus. (From Hegner, after Jordan and 



Evermann.) 



fishes (Fig. 126), characteristic organs of locomotion, broadened to 

 offer the necessary resistance to a fluid medium, and bilateral sym- 

 metry, a common corollary of well-developed powers of locomo- 

 tion. Such invertebrates as aquatic insects and some moUusca, 

 and most aquatic vertebrates are included here. The fishes are 

 excellent examples. 



The Abyssal Realm. The ocean affords still other examples of 

 adjustment to the physical environment in the peculiar fauna of 

 the deep sea. This environment includes depths below 100 fath- 

 oms, where light does not penetrate. The pressure is great, in- 

 creasing at the rate of one ton per square inch with every thousand 

 fathoms, and the temperature is low, near the freezing point in the 

 open ocean. Because of the absence of light plants do not grow, 

 and animals must subsist by eating each other and the things 

 which settle from above. It seems only natural that conditions so 



