1 6 ANIMAL AGGREGATIONS 



as habits become sedentary, and is also more marked in relatively sim- 

 ple animals having strongly developed skeletal parts, as the sponges, 

 hydroids, bryozoans, and tunicates. 



I. Primary colonies arise as the result of division in which the 

 smaller pieces remain together, or as a result of budding in similar 

 fashion. 



1. Homomorphic colonies result when the divisions are equal and 

 all members of the colony are similar, as in Synura, Carchesium, and 

 Salpa chains. Such colonies as Zodthamniiim may represent true 

 societies, since all individuals may contract if one is stimulated, and 

 so all may escape harm; while Carchesium does not, and so is placed 

 in the present category. 



2. Heter amorphic colonies are formed when the divisions are un- 

 equal, as is the case with the strobila of the Scyphozoa, or during the 

 processes of asexual reproduction of certain worms, such as Autolytus. 



II. Secondary colonies, or concrescence colonies, arise by the sec- 

 ondary union of individuals which are entirely separate for at least a 

 brief period. 



1. Concrescence colonies having a genetic basis, in that the individ- 

 uals composing the colonies originated from the same mother, are 

 shown in Proteriodendron, Dinobryon, and secondary Salpa chains. 

 The fact that identical or related forms have survived and can live as 

 separate individuals indicates that these animals are able to live 

 without the small and perhaps accidental benefit arising from their 

 communal life. 



2. Concrescence colonies without a genetic basis are those in which 

 the animals that later become attached together in one colony are 

 not descendants of the same mother. These commonly occur in ses- 

 sile animals, such as the ascidians, sea anemones, sponges, oysters, 

 and Mytilus. If no organic union takes place, causing a real fusion 

 between the different animals composing the colony, then the asso- 

 ciation remains accidental. 



Beta. Associations of free individuals. 



I. Primary associations arise through asexual or sexual reproduc- 

 tion when individuals descending from the same parent or parents 

 remain near the place of origin and form an aggregation which varies 



