154 



GENERAL PRINCIPLES OF ZOOLOGY 



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

 tialh' fusing together; secondly, hy iudiviiluals, fornied liy di\ision and 

 budding, remaining united wilh one anotlier instead of separating. The 

 first is extremely rare. 



Colony Formation by Fusion. — IMany Protozoa fuse mth one 

 another and form larger bodies in which the individual animals can still 

 be recognized. Among the metazoa, Diplozoon paradoxuni (fig. no) is 

 tlie only ease known where two animals (Diporpa-), arising fronr ilifferent 

 eggs, normally unite into a doul>le animal, which recalls certain double 

 monsters, as the Siamese tmns. 



Fig. no. — Development of Diplozoon parjdoxiim (from Boas'), (i") T.;irv;i, from 

 which comes {2) 'Diporpa.' (3) T\\"0 lMporp;i; uniting. {4) Tlie Diporpa^ lia\o 

 united into Piplozoon. hi, motUh; <?, digestive tract; /;, posterior adhering apparatus; 

 l\ 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 docs not separate from the parent. 

 The colonies of marine hydroids and corals (,1'igs. 04, 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 consideral)le degree of community of functions. Stimuli 

 which aif'ect one individual are transmitted to the others of the colony; 

 thus movements in common are rendered possible. In a sinrilar 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 indi\ii.lual of a higher order; the same process which led to the 

 formation of multicellular organisms is repeated. J^st as there the 

 elementary organisms, the cells (indiv iduals of the fu'st 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. — ^\'hen a whole is made up of numerous eiiiuvalent 



