6o ABYSMAL ZONE. 



become adapted to locomotion over a hard surface, and to 

 support against gravity upon this surface. From types 

 which have been so adapted in the past originated all the 

 land animals, for the same problem in more pressing degree 

 has to be solved in them. 



In marked contrast to these are the sedentary group, 

 which are never found on land and remain neritic. 



The katantic sub-zone resembles in most respects the 

 littoral but there is great variety and diversity in so large 

 an area. This zone in a general way has the greater 

 proportion of our valuable food-fishes, together with great 

 numbers of bivalve and univalve Mollusca and Crustacea. 

 Co^lenterata, such as corals and zoophytes, are in great 

 profusion and all the classes of marine fauna are well 

 represented {Nephrops and Raia are examples). 



{c) Abysi7ial Zone, — The Abysmal zone extends from 500 

 fathoms downwards to the greatest depths of the sea. The 

 physical conditions of this zone are unique. Below 500 

 fathoms it is practically certain that no hght penetrates, 

 hence the abysmal zone, so far as natural light is concerned, 

 is in eternal darkness. The pressure increases rapidly with 

 the depth so that "at a depth of 2500 fathoms the pressure 

 is, roughly speaking, two-and-a-half tons per square inch." 

 The greatest storms never affect this zone, hence there is 

 perpetual stillness. The temperature varies enormously 

 but is always lower than that of the surface-water, in many 

 cases very low indeed. This is probably due to extremely 

 slow but widespread polar currents which make their way 

 along the bottom towards the equator. 



No plants can live in this zone for there is no sunlight, 

 but the pelagic life far above appears to shed downwards a 

 continual rain of shells and dead organisms. These former 

 are found in vast numbers in some parts of the ocean. The 

 floor consists of at least three important sediments called 

 ooze. The Red mud is found widely scattered in the 

 greatest depths. It contains the siliceous remains of Radio- 

 larian and diatom shells. Globigerina ooze occurs in 

 shallower water (2000 fathoms upwards) and is characterised 

 by numbers of calcareous shells of Globigerina and other 

 Foraminifera. Pteropod ooze appears to occur at depths of 

 about 1500 fathoms upwards in certain tropical regions. It 



