60 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 Afol/usca and Crustacea. 
Calenterata, such as corals and zoophytes, are in great 
profusion and all the classes of marine fauna are well 
represented (Vephrops and Raia are examples). 
(c) Abysmal 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 light 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 Glodigerina and other 
foramintfera. Pteropod ooze appears to occur at depths of 
about 1500 fathoms upwards in certain tropical regions. It 
