250 THE LIFE OF CRUSTACEA 



a 'species of sessile Barnacle (Balanus) which is 

 cultivated in Japan for use as manure. The method 

 of culture has been described by Professor Mitsukuri. 

 Bunches of bamboo "collectors," like those used 

 for the collection of oyster-spat, are fixed into the 

 ground on tidal flats. After two or three months 

 they are taken up, and the Barnacles with which 

 they have become covered are beaten off and sold 

 for use as manure. 



Apart from their direct utility, however, the Crus- 

 tacea are indirectly of great importance as providing 

 a large part of the food-supply of marketable fishes. 

 From this point of view, a study of the habits and 

 distribution of the commoner species may be of 

 practical value in throwing light on the migrations 

 and other obscure points in the life-history of the 

 fishes that prey upon them. As an example of this, 

 we may refer to some investigations on the Mackerel 

 fishery recently carried out by the naturalists of 

 the Marine Biological Association at Plymouth. In 

 the spring and early summer months the Mackerel 

 migrate into inshore waters for the purpose of 

 spawning. During this period the fish congregate 

 in shoals at the surface of the sea, and are captured 

 in drift-nets. The extent of this " shoaling " varies 

 greatly from year to year, and determines whether 

 the season shall be a profitable one for the fishermen 

 or not. When shoaling, the fish feed exclusively on 

 plankton, consisting largely of Copepoda, and it has 



