(ECOLOGY. 



165 



Colony Formation by Fusion. Many Protozoa fuse with one 

 another and form larger bodies in which the individual animals 

 can still be recognized. Among the multicellular animals, that of 

 Diplozoon paradoxum (fig. 109) is the only case known where two 

 animals (Diporpa), sprung from different eggs, normally unite 

 into a double animal, which recalls certain double monsters, as, for 

 example, the Siamese twins. 



FIG. 109. Development of Diplozoon parailojrum. (From Boas.) (1) Larva, from 

 which comes (2) 'Diporpa.' (3) Two Diporpee uniting. (4) The Diporpae have 

 united into Diplozoon. m, mouth ; rf, digestive tract; /, posterior adhering appa- 

 ratus; b, ventral sucking-disc, which serves for attachment to the dorsal cone, r. 



Colony Formation by Incomplete Division and Budding. In 



general it can be said that the important instances of colony 

 formation rest upon incomplete asexual reproduction. An animal 

 forms new individuals by division or by budding, but the process 

 is not completed since the new generation does not separate from 

 the parent. There remain connexions of tissue uniting the buds 

 with the mother or the sisters with each other. The colonies of 

 marine hydroids and corals (figs, 91, 206) may consist of thousands 

 of individuals which, by repeated incomplete budding or division, 

 have sprung from a single sexually produced mother animal. 



Community of Functions. In the majority of cases the con- 

 nexion of the tissues results in a considerable degree of community 

 of functions. Stimuli which affect one individual are transmitted 

 by common nerves to the others of the colony; thus movements in 

 common are rendered possible. In a similar way the food captured 

 and digested by one animal serves for all. On account of the 

 community of its functions, a colony appears like a unified whole, 

 like an individual of a higher order; the same process which led to 

 the formation of multicellular organisms is repeated. Just as 

 there the elementary organisms, the cells, are united into a single 

 animal, so here the single animals are united into a colony. 



Polymorphism. When a whole is made up of numerous- 

 equivalent parts, the conditions for division of labor are present. 

 Instead of the functions of the entire organism being distributed 



