334 ELEMENTS OF COMPARATIVE ZOOLOGY. 



nourishment for the whole colony; (2) the protective hy- 

 dranths (p) which lack mouths, but which are richly pro- 

 vided with nettle-cells; and (3) the reproductive hydranths 

 (r), the sole function of which is the reproduction of the 

 species. In the Siphonophores this differentiation is carried 



FIG. 147. Part of a colony of the hydroid, Hydractinia^ an illustration of 

 polymorphism. /, feeding individuals ; p, protective individuals ; r, re- 

 productive individual. 



still farther (p. 307), for here seven different forms may be 

 developed. When there are but two different forms in the 

 history of the species it is called dimorphic (from the 

 Greek meaning two forms) ; if more than two, the species 

 is polymorphic. 



Besides the di- or polymorphism produced by budding, 

 similar conditions may arise in other ways. Thus fre- 

 quently we find sexual dimorphism, in which the male and 



