142 CKAYFISU, LilliSTEKS;, SPIUERS, AND INSECTS 



supply, while at the sunie time it tifl'onls pi-i.ilectiun to the 

 crab. These animals tlieii are of mutual beiieht to each 

 other, and the sea anemone is the inessiiiuic or commensal 

 icon, together; menm, table) of the crab. 8uch a rela- 

 tio!i between two aihmals is called coinmensalisni. 



The same relation exists between the Hydractinia and the 

 hernut crab. Certain species of sp(jnges "are never found 

 gi-Qwing; except on the backs or legs of certain crabs.'' 

 This is evidently a case of conimensalism, for the sponges 

 conceal the crabs fi'oiu their enemies and, in turn, are 

 transjjoi'ted to new supj)lies of footl. 



Economic importance of the crustaceans. — Many of the 

 crustaceans form au important source of food supply. 

 Many millions of lobsters are taken along the Atlantic 

 coast annually. The value to the fishermen of the lobsters 

 taken off the coasts of the United States is estimated at 

 luore than one million dollars. The blue-cralj fisheries 

 amount to a product aggregating nearly half a million dol- 

 lars. Many factories for canning young prawns, which 

 are usually sold as shrimps, are located along the Gulf 

 coast. The shrimp industry of the Pacific coast is very 

 large. 



In a discussion of the economic importance of the 

 crustaceans, the smaller forms must be taken into account, 

 for they constitute tlie principal food of most of our fre.sh- 

 watei' fishes while young. These minute crustaceans mul- 

 liply very rapidl\' and become exceedingly abundant. 

 Large areas of wafer, hundreds of miles in extent, m the 

 yVtlantic Ocean are sometimes colored red Ijy the swarms 

 of these minute orga,iiisuis. At such times fishes congi-e- 

 gate in large numbers to feed upon Ihem, and even whales 

 find that these tiny creatui'es furidsh an abundant food 



