158 THE DISTRIBUTION OF 
plankton. Theimportance of the waste of the land may 
be realized if we think of the number of marine animals 
which find food in the mud-deposits which accumulate 
round the mouths of rivers and estuaries. If the water 
be fairly clear, such estuaries are specially favourable 
to shell-fish like mussels and oysters ; to many marine 
worms, which in their turn feed many fishes ; to many 
small crustacea, which also feed fish ; and so forth. 
This land-waste has only a limited seaward extension, 
and is not available as a source of supply either for the 
pelagic or for the abyssal animals. 
The fixed algae can only grow where some firm sur- 
face to which they can attach themselves exists, and 
where the sunlight is strongly felt. They are therefore 
limited to the margin of the lands, but there form rich 
feeding-grounds. The Laminarian zone, which lies just 
beyond low-tide mark, has always been known as 
a rich collecting-ground. It contains many small forms 
—crustacea, molluscs, worms, coelenterates, and so 
forth—perfectly adapted to life among the long brown 
fronds and about the branching roots. At high tide 
this zone is covered with a sufficient depth of water 
to be available as a pasture ground even for the larger 
fish, and here therefore shore animals cluster, and the 
fisherman reaps rich harvests. 
Finally, in the shore waters minute algae swarm. 
In their habits and adaptations most of these are 
pelagic, that is apparently independent of the presence 
of a substratum, but the fact that there is considerable 
difference between the phytoplankton of the shore 
water and of the open sea, suggests that these forms 
are in some way indirectly affected by the proximity 
of the sea-floor, or by the waste of the land. Diatoms, 
especially, which are eagerly consumed by many 
