54 FOOD FISHES 



To the same class of molluscs belong Mussels and Scallops, 1 

 Cockles and Whelks ; the mussels are attached to rocks by 

 their beards, but the cockles and scallops are able to move 

 about slowly. The molluscs which have two shells joined 

 together by a hinge are called bivalves ; those with only one 

 shell univalves. To the former class belong oysters and 

 scallops, and to the latter cockles and whelks. 



Fish, then, are dependent on their environment. Some 

 inhabit the sandy or rocky floor of the sea, others swim in 

 the middle or surface waters, but all kinds have to seek out 

 those places where their food is abundant, and where the 

 temperature of the water and its salinity and density are such 

 as suit them. So that a great variety of conditions is matched 

 by a corresponding variety of fish. Not only so, but at different 

 stages of their development they require entirely different con- 

 ditions ; thus the eggs of plaice, to take only one instance, float 

 upon the'surface of the sea, but the larvae in their final stage 

 have to fall to the bottom in very shallow water, and here they 

 develop into fish, and henceforth live upon the floor of the 

 sea, though, as they grow bigger, they gradually seek deeper 

 and deeper water. 



The currents of the sea, too, play an important part in the 

 life-history of fishes, for all eggs and larvae (and sometimes 

 even full-grown fish) are entirely at their mercy, and the number 

 of fish produced depends largely on whether the currents carry 

 the eggs or larvae to places where the temperature of the 

 water, and other conditions, are suitable for their development, 

 and where there is an abundance of food for the young fishes. 



These great movements of the sea also serve another purpose ; 

 they keep the water aerated and sweet, and this is one reason 

 why open seas, such as the North Sea, are richer in fish than 

 enclosed areas such as the Black Sea and Mediterranean. 



1 Scallops are plentiful on the coast of Palestine, hence pilgrims and 

 palmers used to wear them on their caps to show they had visited the 

 Holy Land. 



The summoned Palmer came in place, . . . 



The scallop shell his cap did deck. Marmion. 



