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zoophytes and polyzoa growing there may appear as if they 

 were thickly dusted over with such minute mussels. Some 

 counts made recently showed that extensive areas of sandbanks 

 contained little cockles in such numbers as several hundreds per 

 square foot, while the number of small mussels on a square 

 foot of suitable " skear " ground may run into the thousands. 

 Such invertebrate communities are the feeding grounds of 

 shrimps, crabs, starfishes, and young fish of various kinds 

 (plaice, flounders, dabs, soles, cod, whiting, sprats, etc.). Here 

 small plaice feed greedily upon Copepods, small worms, little 

 periwinkles, and very small bivalve molluscs, while the larger 

 fish eat the small Mactra, Scrobicnlaria, and cockles that are 

 nearly everywhere present in the sand. In the spring and early 

 summer months the temperature of the water on the nursery 

 grounds rises several degrees higher than it is offshore ; the 

 tides run more strongly and so distribute the dissolved food 

 substances used by the Diatoms, Flagellates, and Dinoflagel- 

 lates. The sunlight penetrates to the bottom layers of water, 

 overlying the sand, better than it does offshore, and this is 

 favourable to the nutrition of the microscopic plants and 

 plant-animals. The latter are then eaten by the shellfish and 

 the smaller Crustacea and worms, which are, in their turn, 

 eaten by the small fishes, large fishes, and invertebrates. As 

 fast as the fundamental food substances in solution are taken 

 from the water by the organisms that exhibit the vegetable 

 mode of nutrition they are renewed by the drainage coming 

 down from cultivated land and from domestic sewage entering 

 the estuaries and then the open sea. The higher temperature 

 on the nursery grounds accelerates the rate of growth of all 

 animals living there. Further, the conditions, as regards 

 temperature and density of the water, are more variable than 

 they are offshore because of the more rapid tidal streams, 

 freshets entering the estuaries, and the greater agitation of the 

 water by wind action. This variability in external conditions 



