278 PRINCIPLES OF. ANIMAL BIOLOGY 



Colonies. — Colony formation has been considered in the chapter on 

 cell aggregation, and it is not necessary to go into detail here. An 

 animal colony is a union of numerous individual animals by an organic 

 body connection. True colony formation results, as pointed out in 

 Chapter V, only from the adherence of individuals produced by fission 

 or budding. It is not accomplished by the coalescence of originally 

 separate individuals; the fusion of separate cells of the slime-molds 

 (Myxomycetes) into a multinucleate streaming mass (the plasmodium), 

 and the secondary union of two individuals of a mctazoon, Diplozoon 

 paradoxum, in a permanent copulation, are not to be thought of as pro- 

 ducing colonies. The marine hydroidicand corals are examples of colony 

 formation by incomplete division aiilt budding. The colonies formed 

 by budding may consist of thousands of individuals which have sprung 

 from a sexually produced ancestor. Frequently in such colonies there 

 is polymorphism, permitting the functions of the entire aggregation to be 

 distributed among the individuals, so that there are reproductive units, 

 units which capture and digest food, protective and sensitive units, etc. 



Societies. — In social animals the reciprocal dependence of the indi- 

 viduals is much less than in colonial forms in that there is communal life 

 but no organic connection. It is probable that the sexual impulse plays 

 an important part in the formation of animal societies. It is known that 

 under the influence of the sexual impulse many animals are drawn 

 together temporarily; for example, sea urchins, many fishes and snakes 

 congregate in numbers during the breeding season. Other forms are 

 drawn together into loose but more persistent groups apparently princi- 

 pally because of the reproductive impulse and for the protection of the 

 j^oung ; herds of gregarious mammals, like the bison and deer, are examples. 



Finally there are very elaborate societies among insects, in which 

 there is pronounced division of labor and polymorphism and thus recipro- 

 cal dependence. Many of the ants and bees live in complex communities 

 composed of males and females (drones and queens) and others with 

 degenerate female sexual organs (workers). The sexual forms give 

 rise to the next generation, while the workers care for the home, provide 

 the food, and serve for defence. Sometimes the workers are also divided 

 into castes with a division of labor, a common division being that between 

 true workers and soldiers, the defence of the colony being delegated to 

 the latter. 



Parasitism. — Individuals of different species often stand in close rela- 

 tion to each other because of advantages which they derive from one 

 another or because of something which one furnishes to another. Where 

 one form attaches itself to another and derives all the benefit, the associa- 

 tion is called parasitism. A parasite is an animal which finds its dwelling- 

 place upon or in the body of another animal, the host, and obtains nour- 

 ishment from it. Parasitic forms hav(! thus come into a more or less 



