THE PELAGIAL 233 



do not obstruct the motion and combine the function of aiding sus- 

 pension with that of a rudder. 12 In the swiftest of the swimmers, such 

 as the blue shark and the mackerels, every unevenness of the body 

 is removed, and in the mackerels the body fins fold into grooves when 

 not in use. 



Plankton and nekton. — Pelagic animals may be grouped accord- 

 ing to their ability to swim freely, independent of oceanic currents, or 

 their dependence on the latter; the free-swimming forms are called 

 nekton as distinguished from the passively floating or suspended plank- 

 ton. It is evident that no sharp division between these groups exists, 

 but they merit discussion as including assemblages of animals with 

 much in common. Some modern students use the term seston to apply 

 to all swimming or floating bodies in the water, alive or dead; the 

 non-living seston is then called tripton; the living seston associated 

 intimately with the surface film is called neuston. 



The plankton includes all those animals and plants that drift aim- 

 lessly, i.e., those whose independent movements are insignificant in 

 comparison with the movement of the water. Most of them are small 

 or microscopic, and all microscopic animals which are pelagic belong 

 to the plankton. The converse is not true, as many larger animals 

 with muscular movement are also included in this category, such as 

 the jellyfish Cyanea arctica, which reaches a diameter of 2 m. The 

 giant shark Cetorhinus maximus is so very dependent on the Atlantic 

 currents that it might almost be included with the plankton. In quiet 

 water even small animals may be capable of directed movements, 

 especially vertical ones, such as a nocturnal rise to the surface. A prac- 

 tical definition of plankton includes those forms which are unable to 

 escape the ordinary plankton net by their own movements. Plankton 

 animals may be assorted according to size as nannoplankton, micro- 

 plankton, mesoplankton, and megaloplankton. The minute nannoplank- 

 ton escaped observation until it was discovered in the feeding apparatus 

 of the appendiculates. It is now secured by filtration or centrifuging. 

 The boundary between these several sizes of plankton is an arbi- 

 trary one. 



The nekton includes the free-swimming forms, which are independ- 

 ent of wind and current. Complete independence of currents is reached 

 only by certain fishes, some cephalopods, the marine reptiles, and the 

 homoiothermal marine birds and mammals. Both plankton and nekton 

 include predaceous as well as herbivorous forms, and cannot be dis- 

 tinguished according to their food habits. 



The viscosity of water, which is of such great importance in re- 

 ducing the rate of sinking, is not uniform. It depends chiefly on the 



