CHARACTERISTICS OF ASSOCIATIONS ii 



profusion Similarly, the animal population is extraordinarily 

 dense. 



Abyssal Realm. — This is the area extending from the 

 margin of the continental shelf to the greatest depths and 

 bounded above by the pelagic zone. At the margin of the 

 continental shelf the sea-floor turns abruptly downwards, 

 and the depth of water increases very rapidly. The amount 

 of light penetrating diminishes accordingly, and plant life 

 with all that it implies grows less common and eventually 

 disappears. Wave action no longer makes itself felt and 

 the waters are motionless. It is extremely cold, almost 

 freezing, the sun's influence not extending beyond about 

 1 50 fathoms. Finally, the pressure of the waters is enormous, 

 since pressure in the sea increases by about one atmosphere 

 for every ten metres of depth (the pressure at 5000 metres 

 being over three tons to the square inch). These four 

 factors : darkness, quiescence, cold, and pressure, have had 

 far-reaching effects on the fauna, particularly in the case of 

 deep-sea fishes. The animals are frail and flabby, their 

 colours are extremely simple, eyes are often absent, and when 

 present they are frequently telescopic or provided with an 

 enlarged lens like that of a bull's-eye lantern. There is a 

 tendency to develop feelers or long filaments with a tactile 

 sense, obviously only of use in still waters. Nearly all 

 abyssal fishes are phosphorescent. 



SOME CONSIDERATIONS REGARDING THE ECOLOGICAL 

 STUDY OF THE SHORE 



While a detailed discussion of the associations of the 

 shore is deferred till Chapter III, we may take this oppor- 

 tunity of considering how the ecological method of study 

 may best be appUed to the case of this particular life area. 



The area exposed between tide-marks can usually be 

 split up into a number of life zones. Each of these zones 

 or horizons is characterised by a certain assemblage of 

 forms, some of which are either confined to that horizon 

 or are so marked in form or predominant in numbers as to 



