356 



GENERAL PRINCIPLES 



ming bells. The vibraculae and avicularia of the Bryozoa, are 

 also modified zooids. In these cases, which are characterized 

 as stock polymorphism, the differentiation of individuals occurs 

 in connection with division of labor, and in this special type 

 could only occur in a colony. An analogous kind of polymor- 

 phism occurs in the social Hymenoptera, the bees and ants, and 

 the termites among the Corrodentia. In these societies there 



t 



r. 



FIG. 223. Hydractinia, a polymorphic hydroid. C, Ccenosarc covering the 

 substratum; n, trophic polyps; r, reproductive polyps bearing buds containing 

 ova; t, tentacles. (From Galloway after Hincks.) 



are males, females and workers. The latter are undeveloped 

 females. In some ants and termites there is also a fourth class, 

 the soldier, individuals with exceptionally large heads and 

 formidable jaw. The interdependence of the individuals of 

 these societies is almost as great as that of polyps in a 

 Hydroid colony. 



741. Alternation of Generations. When polymorphic types 

 alternate with each other in successive generations we have the 



