154 



GENERAL PRINCIPLES OF ZOOLOGY 



latter may arise in. two ways: first, by animals, originally separate, par- 

 tially fusing together; secondly, by individuals, formed by division and 

 budding, remaining united with one another instead of separating. The 

 first is extremely rare. 



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 metazoa, Diplozoon paradoxum (fig. no) is 

 the only case known where two animals (Diporpa?), arising from different" 

 eggs, normally unite into a double animal, which recalls certain double 

 monsters, as the Siamese twins. 



FIG. no. Development of Diplozoon paradoxum (from Boas), (i) Larva, from 

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

 united into Diplozoon. m, mouth; d, digestive tract; h, posterior adhering apparatus; 

 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 colony formation rests upon incomplete asexual 

 reproduction, since the new generation does not separate from the parent. 

 The colonies of marine hydroids and corals (figs. 94, 205) 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 connection 

 results in a considerable degree of community of functions. Stimuli 

 which affect one individual are transmitted 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 (individuals of the first order) are united 

 into a single animal (individual of the second order), so here the multi- 

 cellular animals are united into a colony (individual of the third order). 



Polymorphism. When a whole is made up of numerous equivalent 



