64 LIFE IN THE SEA [PART I 



does not continue too long. These effects are produced by the 

 inhibition of water by the tissues ^ In what w^ay the abyssal 

 animals have become adapted to the great pressures Avhich they 

 undergo without apparent inconvenience is difficult to understand. 

 The effects produced on a deep-sea fish when it is suddenly 

 brought up from its habitat are well known. The intestine or 

 stomach is usually projecting from the mouth and the flesh and 

 bones are disintegrated by the release of pressure and the con- 

 sequent expansion of the liquid or gas contained in the tissues. 



In the depths of the ocean the temperature is always uniform, 

 and is nearly the same over vast areas. Over great extents of sea 

 floor the bottom deposit is the same. Currents are very feeble. 

 The composition of the water does not vary. We find then that 

 in accordance with this sameness there is great uniformity in the 

 distribution of the fauna. Some deep-sea species are cosmopolitan 

 and many are very widely and uniformly distributed. 



The migratory fauna. So much for the benthos or bottom 

 living animals and plants of the sea. We have seen that its 

 constituents are the larger sea-weeds and the invertebrates. The 

 nekton or actively locomotory organisms in the sea are almost 

 entirely the vertebrates. These are they which can move about 

 in obedience to their volitions, or in response to instincts, or as the 

 reflex to physical changes. They migrate independently or in 

 opposition to the currents or drifts which carry about the feebler 

 planktonic species. The marine mammalia are the chief among 

 the nektic animals, and these, with the great sharks, roam over 

 extensive tracts of ocean and probably make cruises which are 

 comparable with those of the large ocean tramp steamers. The 

 fishes are the type of the nekton. Some, like the flat-fishes, are 

 often spoken of as semi-sedentary, but it is more probable that 

 even these are nearly always "on the move." Others like the cod, 

 hake, herring, salmon and many others are notorious wanderers 

 and swim over wide areas of sea. The fresh-Avater eel it is now 

 known makes an extraordinary long spawning migration, and, as 

 Schmidt has shewn-, the eels of the north of Europe migrate 



^ Regnard, La Vie dans les Eaux, Paris, 1891. 



2 Schmidt, Rapports et Proces-Verhaux ; Cons. Perm. Int. Exp. de laMer, Vol. v., 

 Copenhagen, 1906. 



