NUTRITION AND METHODS OF FEEDING 189 



to feed within a minute after they are covered by the 

 tide. 



Contrary to what one would expect, the fixed algae are 

 directly utilised as food only to a relatively small extent. It 

 is true that a certain number of shore creatures (several 

 molluscs, sea-urchin) browse upon seaweeds — ^witness, for 

 instance, the feeding tracks of the small limpet Helcion 

 pellucidiim on the stems and fronds of Laminaria — but a 

 great proportion of the fixed plants of the sea-shore are 

 allowed to live out their lives with only a comparatively 

 trivial amount of interference from animals. When esti- 

 mating the importance of the fixed plants of the littoral 

 as a source of food, allowance must be made, however, for 

 the quantities of organic debris they provide for the use of 

 numerous creatures such as worms {Arenicola, Cirratulus^ 

 and numerous Oligochaets) and Crustacea {e.g. Corophium 

 and other Amphipods) which burrow in the sandy mud 

 of the sea-floor. Devoid of any means of attacking the 

 living weed, these creatures will make good use of it after 

 its reduction to detritus. Looking at the matter from 

 another standpoint, we may say that these detritus feeders 

 perform a valuable service as scavengers by preventing the 

 over-accumulation of decaying matter in the sea-floor and 

 by aiding the work of bacteria. 



The larger algae are also useful in intercepting floating 

 organisms. These, on coming into contact with the weed, 

 remain adherent to the gummy surface of the fronds and 

 are then readily scraped off by animals such as periwinkles. 



If the seaweeds are not consumed directly to any great 

 extent, the plankton, on the other hand, is exploited in the 

 intensest possible way. The floor of the sea-shore has been 

 likened to an immense sieve through which the microscopic 

 contents of the water disappear. Barnacles sweep it clean 

 the carpet of bivalves siphons food from the turbid waters, 

 periwinkles search every square inch of the rock and scrape 

 it clean, coelenterates and polyzoa spread their tentacles 

 and take toll of passers-by (see Pearse, 1913). 



We now turn to a more systematic study of the feeding 



