POLYMORPHISM 



99 



hydroid which begins the formation of a new colony by 



budding. A hydroid, thus, may be polymorphic; Hydra is not. 



Other types of hydroids present different grades in complexity 



of this phenomenon of polymorphism, some individuals being 



FIG. 39. Stephalia corona. A siphonophore colony illustrating polymorphism. 

 A, aurophore, a modified medusa; G, secondary gastrozooid or feeding polyp; 

 T, tentacle of gastrozooid; PG, primary gastrozooid, N, nectophore, one of the 

 locomotor individuals of the colony and with the general form of a medusa; P, 

 pneumatophore or air sac. (From Lankester after Haeckel.) 



differentiated for purposes of locomotion, others for protection 

 and defence, and others for reproduction. The highest devel- 

 opment of this coelenterate polymorphism is found in the group 



