iS95. RESULTS OF '^ CHALLENGER" EXPEDITION. 31 



conclusions that have been at least partially the result of the 

 Expedition. 



Lastly, the Expedition conclusively established the existence of 

 life down to the deepest abysses, and did much to catalogue the kinds 

 of animals present in the different regions of the sea. 



Combining the actual results of the " Challenger" with those of 

 later investigations, most of which, indeed, were due to the stimulus 

 given by the " Challenger " results, we are able to define the fauna of 

 the sea. There are three great groups of marine inhabitants : a 

 group that drifts, a group that swims, and a group that is anchored. 

 The first group, or Plankton, consists of all those frail forms that float 

 in the waters devoid of the power of movement or at the least of the 

 power of movement against tide and current. The Nekton are those 

 animals capable of swimming against currents, and so of migrating 

 from place to place with or against the currents as they choose. The 

 Benthos are those animals and plants that are fixed to the bottom, or 

 that can crawl over the bottom only for short distances, and, there- 

 fore, neither migrate at will nor are carried about by the set of 

 currents. The Plankton, Nekton, and Benthos form three well-marked 

 communities of organisms, each having its own characteristic forms, 

 and, naturally, its own seasonal variations. The Benthos and the 

 Plankton can be studied more fully, as in their case the variable and 

 elusive factor of " will " does not come into operation. 



It is, of course, to be noted that both the Nekton and the Benthos 

 contribute to the Plankton. The young forms of strongly swimming 

 creatures like fishes or cephalopods are carried about by the currents, 

 and, at certain seasons, compose a large part of the Plankton ; while 

 similarly the larval stages of Benthos, such as starfish, mollusks, and 

 ascidians, are also Plankton. 



The regional classification of marine life is more difficult. The 

 simplest region is the Pelagic zone. This extends all over the seas, 

 from the coasts to the great oceans : its limits are difficult to define, 

 but may be taken as practically the depth to which strong sunlight 

 penetrates. Its inhabitants are Plankton and Nekton, and the great 

 character separating it from deeper waters is the presence of abundant 

 plant life. The second great zone is the Neritic zone which extends 

 from the coast to a depth of about five hundred fathoms, excluding 

 the superficial Plankton, and including the region left bare by the 

 tides. It corresponds to the littoral region of Moseley, with the addi- 

 tion of the shallow banks in seas far from coasts. Its inhabitants 

 are chiefiy Benthos, and the great character is dependence upon the 

 coast. The creatures of the pelagic zone differ little all over the 

 surface of the sea, and the inhabitants of the deeper waters vary still 

 less from ocean to ocean, but the littoral zone of each coast has its 

 own peculiar features. The Abysmal zone extends from a depth of 

 500 fathoms down to the bottom of the deepest oceans. Its inhabitants 

 are chiefly, if not entirely, Benthos and Nekton. 



