HISTORY AND NATURAL HISTORY 41 



Within these communities aggregations of animals 

 occur for a variety of reasons. Their nature can best 

 be shown by a series of illustrations. 



One variety of aggregations is that of colonial 

 forms, in which many different so-called individuals 

 remain through life permanently attached together. 

 In the simplest cases all the individuals are alike. 

 Each possesses a mouth and food-catching tentacles, 

 and each feeds primarily for itself, although food 

 caught by one individual may be shared with others 

 near by. In more complex forms some individuals 

 have the mouths suppressed, and receive all their 

 food from those that do take food. They have be- 

 come specialized as bearers of batteries of stinging 

 cells; they strike actively when the colony is touched, 

 and their stinging cells explode so effectively as to 

 give protection to the colony. Other individuals in 

 the same colony bear medusa-like heads which break 

 away and swim off, producing eggs and sperm, dis- 

 tributing them as they drift. Here is certainly a divi- 

 sion of labor though these colonial animals are 

 never rated as social. 



Various modifications of such colonial animals 

 are found particularly among the colonial protozoa, 

 sponges and the coelenterates; they also occur higher 

 in the animal kingdom, even among the lower 

 chordates, the great phylum to which man himself 



