MUTUAL AID AND COMMTTNAL LIFE AMONG ANIMAL.S 371 



through the water. The Reniora injures llie shark in no way 

 save, perhaps, by the shght check its presence gives to the 

 shark's speed in swimming. 



In the mouth of the menhaden (Brevoortia tyratuius) a small 

 crustacean (Cynioihcu prwgusUitor) is almost always ])resent, 

 always resting in the front of the lower jaw. This arrange- 

 ment is of advantage to the crustacean, but is a matter of in- 

 difference to the fish, I^atrobe, who first described this fish, 

 compares the crustacean to the pra>gustator or foret aster of 

 the Roman tyrants — a slave used in prevention of poisoning. 



Whales, similarly, often carry barnacles about with them. 

 These barnacles are permanently attached to the skin of the 

 whale just as they would be to a stone or wooden pile. Many 

 small crustaceans, annelids, moUusks, and other invertebrates 

 burrow into the substance of living sponges, not for the ])urpo^c 

 of feeding on them, but for shelter. On the other hand, the 

 little boring sponge (Cliotm) burrows in the shells of oysters 

 and other bivalves for protection. These are hardl\' true case.>3 

 of even that lesser degree of mutualh' advantageous associa- 

 tion which we are calling commensalism. But some species of 

 sponge "are never found growing except on the baclvs or legs 

 of certain crabs.'' In these cases the s])onge. with its many 

 plantlike branches, protects the crab by concealing it from 

 its enemies, while the sponge is benefited by being carried al)out 

 by the crab to new food supplies. Certain sponges and polyps 

 are always found growing in close association, though what the 

 mutual advantage of this association is has not yet been foiuitl 

 out. 



Among the coral reefs in the South Seas there lives an 

 enormous kind of sea anemone or poly]^. Individuals of this 

 great polyp measure two feet across the disk when fully ex- 

 panded. In the interior, the stomach cavity, which com- 

 municates freely with the outside \)y means of the large mouth 

 opening at the free end of the polyp, there may often be found 

 a small fish {Aj?iphi prion pcrculn). That this fish is j)urp()sely 

 in the gastral cavity of the polyp is proved by the fact that 

 when it is dislodged it invariably returns to its singular lodging 

 place. The fish is brightly colored, being of a brilliant vermilion 

 hue with three broad white cross bands. The discoverer of, 

 this peculiar habit suggests that there are mutual benefits to 

 fish and polyp from this habit. "The fish being conspicuous, 



