158 THE DISTRIBUTION OF 



plankton. The importance 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 sunHght 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, moUuscs, 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 



