160 ANIMAL INDIVIDUALITY 



are left, through which the two cell-bodies enjoy protoplasmic 

 communication. This type of organization is found in all the 

 higher green plants. In the third type (Coenocytic) the nucleus 

 alone divides, and the final result is a coenocyte a single over- 

 grown cell with a single cell-wall and many nuclei. This plan nas 

 been adopted by the Siphoneae (p. 89). 



It is obvious that the first method is the most primitive and 

 will be most generally practised by unicellular organisms ; but 

 whereas it has been abandoned by the higher plants, it seems to 

 have been retained by the higher animals. Almost the only 

 difference between the division of a protozoan and a metazoan 

 cell lies in the fact that the two daughter-cells separate in the 

 one case, cohere in the other. The essential separateness of the 

 cohering cells is well seen in the collar-cells of simple Calcareous 

 Sponges like Clathrina ; here indeed there is even no continuity of 

 coherence during normal life (p. 93). 



Similar if less strikingly separate cells can be seen in many 

 other groups of multicellular animals, and there can be very little 

 doubt that the first method of division was employed by the 

 common ancestor of all Metazoa 1 ; true party- walls like those of 

 filamentous plants do not exist in animals, and animal syncytia 

 (tissues formed by the coenocytic method) are undoubtedly 

 secondary. 



We must now try and see what these facts mean. In the 

 filamentous type the units are still homologous, as units, with the 

 original units we called cells (p. 56) ; but they have sacrificed a 

 considerable amount of independence. The whole mode of 

 division by which they arise is an obvious adaptation to a state 

 of existence where each is to be part of a continuous w r hole. 



1 It is more than probable that Sponges have an ancestry quite 

 separate from the rest of the Metazoa : if so, then the common 

 ancestor of Sponges employed, though quite independently, the same 

 method as the ancestor of the Metazoa proper. 



