142 CRAYFISH, LOBSTERS, SPIDERS, AND INSECTS 



supply, while at the same time it affords protection to the 

 crab. These animals then are of mutual benefit to each 

 other, and the sea anemone is the messmate or commensal 

 {con, together; mensa, table) of the crab. Such a rela- 

 tion between two animals is called commensalism. 



The same relation exists between the Hydractinia and the 

 hermit crab. Certain species of sponges " are never found 

 growing except on the backs or legs of certain crabs." 

 This is evidently a case of commensalism, for the sponges 

 conceal the crabs from their enemies and, in turn, are 

 transported to new supplies of food. 



Economic importance of the crustaceans. — Many of the 

 crustaceans form an 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 

 more than one million dollars. The blue-crab 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 the principal food of most of our fresh- 

 water fishes while young. These minute crustaceans mul- 

 tiply very rapidly and become exceedingly abundant. 

 Large areas of water, hundreds of miles in extent, in the 

 Atlantic Ocean are sometimes colored red by the swarms 

 of these minute organisms. At such times fishes congre- 

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

 find that these tiny creatures furnish an abundant food 



