494- GENERAL CONSIDERATIONS 



557. Colonies. — When the parents and offspring remain in physical 

 continuity, as in colonial hydroids, many anthozoan polyps, bryozoans, 

 tunicates, and some other forms, what is known as a colony is produced. 

 In such a colony there is often division of labor and dependence of one 

 upon another. It may even result in the functioning of the whole as an 

 organism made up of many individuals. 



558. Societies. — If the offspring do not remain in physical contact 

 but become separate individuals and yet these associate together, the 

 group is termed a society. A society may, as in the case of various worms 

 and barnacles, involve no dependence and no division of labor, but it is 

 also possible to have division of labor and polymorphism within a society, 

 such as in the case of ants and bees. Societies are not always the descend- 

 ants of a single pair but may include unrelated individuals of the same 

 species brought together by a social instinct. 



559. Associations of Animals of Different Species. — Animals of 

 different species are also found associated together. This involves 

 relationships of various degrees of intimacy and with a varying distribu- 

 tion of benefits and injuries. The different terms which have been 

 applied have been used in such varied senses that what they mean in 

 any given place can be determined only by recourse to definitions or 

 inferences from the facts presented. For this reason the examples of 

 certain associations given here may be found elsewhere under dif- 

 ferent names. The terms used are aimed to bring out various degrees of 

 relationship. 



560. Gregariousness. — Gregariousness is a term applied to the 

 tendency of animals to gather together in one place. If these are of the 

 same species, a society may result; but it may involve different species 

 and be due to the presence in that place of desirable conditions of exist- 

 ence, including food, shelter, moisture, and other environmental factors. 

 Such a relationship is exhibited when in a marsh are gathered together a 

 variety of marsh-loving organisms. It is also exhibited when birds of 

 many species gather on an island in the ocean where they find conditions 

 favorable for nesting and the rearing of young. In the highest animals, 

 including those which possess intelligence, gregariousness may be the 

 result of a desire for companionship, which also may be mingled with a 

 feeling of safety in the presence of numbers, even though the individuals 

 may be of different species. This safety may be a real factor if in the 

 gregarious assemblage there are individuals which by their sounding of 

 an alarm give warning of danger to the others. 



561. Epizoic Associations. — The word epizoic implies the living of 

 one animal upon another, not as a parasite but as it might hve on any 

 nonliving object. Colonial protozoans and hydroids which ordinarily 

 attach themselves to rocks and other objects in the water may live upon 

 the shells of mollusks, crustaceans, and other marine forms and thus 



