SOCIAL LIFE OF ORGANISMS 431 



for its sustenance. But among these higher algae the growth 

 of the mass of cells assumes a rather definite form, and certain 

 of the groups of cells become specialized as anchorage organs, 

 others become specialized as reproductive organs, and so on ; 

 and it takes all of these together to make up a complete life. 



Fig. 228. Colony of Hydractinia 



In this colonial animal (related to the jellyfish and to corals), as in many others, there 

 are distinct kinds of individuals, called hydranths. a, vegetative, or food-getting, 

 hydranths, which take in and digest food for the whole colony ; 6, vegetative hydranths 

 in various stages of contraction ; c, protective, or fighting, hydranths, which bear large 

 numbers of nettling cells ; d, reproductive hydranths, male and female, which throw off 

 sperm cells and egg cells respectively ; e, buds, or undeveloped hydranths. (Photograph 

 from model in American Museum of Natural History) 



If we consider the whole plant as an individual, we see that 

 it is quite possible for a single plant to continue its life without 

 relation to any others of the species. In general this is true 

 of all the plants, from the lowest to the highest. 



458. Differentiated cells. When we pass from the one-celled 

 animals to the sponges (Fig. 227), we find that while the life 



