MORPHOLOGY. 



407 



ing two forms) ; if more than two, the species is polymor- 

 phic, no matter whether the forms are colonial or whether 

 they lead distinct lives. 

 Besides the di- or polymorphism produced by budding, 



FIG. 175. Part of a colony of the hydroid, H ' ydractinia, an. illustration of poly- 

 morphism. /, feeding individuals; p, protective individuals; r, reproduc- 

 tive individual. 



similar conditions may arise in other ways. Thus fre- 

 quently we find sexual dimorphism, in which the male and 

 female of the same species are greatly different in their 

 appearance. An example of this is familiar in the can- 

 kerworm-moths, the male of which is winged, the female 

 wingless. More striking cases are furnished by many 

 Crustacea where the female has become so aberrant that 

 in the adult all arthropod features may have disappeared 

 (fig. 176). 



