336 ANIMAL STUDIES 



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 



FIG. 201. Remora, with dorsal fin modified to be a sucking plate by which the fish 

 attaches itself to a shark. 



wooden pile. Many small crustaceans, annelids, mollusks, 

 and other invertebrates burrow into the substance of living 

 sponges for shelter. On the other hand, the little boring 

 sponge (Gliona) burrows in the shells of oysters and other 

 bivalves for protection. Some species of sponge " are never 

 found growing except on the backs or legs of certain 

 crabs." In these cases the sponge, with its many plant-like 

 branches, protects the crab by concealing it from its ene- 

 mies, while the sponge is benefited by being carried about 

 by the crab to new food supplies. 



Small fish of the genus Nomeus may often be found 

 accompanying the beautiful Portuguese man-of-war (PJiy- 

 salia) as it sails slowly about on the ocean's surface (Fig. 

 202). These little fish lurk underneath the float and among 

 the various hanging thread-like parts of the Pliysalia, 

 which are provided with stinging cells. 



In the nests of the various species of ants and termites 

 many, different kinds of other insects have been found. 

 Some of these are harmful to their hosts, in that they feed 

 on the food stores gathered by the industrious and provi- 

 dent ant, but others appear to feed only on refuse or use- 

 less substances in the nest. Some may even be of help to 

 their hosts. Over one thousand species have been recorded 

 by collectors as living habitually in the nests of ants and 

 termites. 



