THE CCELENTERATES 



205 



be division of labor among the individuals. This has been 

 noted in the honeybee (p. 63), where the females of the 

 colony are of two kinds, queens and workers. Other social 

 insects, such as ants, wasps, and 

 termites, exhibit similar differences 

 among the members of a colony. 

 Division of labor likewise exists in 

 communities of human beings, but 

 the structural differences are not as 

 great as in many of the, lower ani- 

 mals. When division of labor oc- 

 curs among the members of a colony, 

 the form of the individual is usually 

 modified so as to be suited to the 

 function it performs. A colony con- 

 taining two kinds of members is said 

 to be dimorphic; one containing 

 more than two kinds, polymorphic. 

 Some of the most remarkable cases 

 of polymorphism occur among the 

 hydroids. The " Portuguese man- 

 of-war " (Fig. 121), for example, 

 consists of a float with a sail-like 

 crest from which a number of polyps 

 hang down into the water. Some 

 of these polyps are nutritive, others 

 are tactile; some contain batteries 

 of nematocysts, others are male re- 

 productive zooids; and still others 

 give rise to egg-producing medusae. 



Alternation of Generations. — Most of the relatives of Hydra 

 live in the sea. Some of them are much branched, plantlike 

 animals that look like a colony of Hydras attached to a central 

 stem. These hydroids are of particular biological interest 

 because of their method of reproduction. Reference to Figure 



Fig. 121. — The Portuguese 

 man-of-war, a colonial ccelen- 

 terate. (After Agassiz.) 



