THE WONDERS OF LIFE 



pliologically to real nucleated cells. By means of adap- 

 tation to the most varied conditions and the inheritance 

 of the properties thus acquired such a variety of uni- 

 cellular forms has been evolved in the course of millions 

 of years that we can distinguish thousands of living 

 species, both of plasmodomous protophyta and plas- 

 mophagous protozoa. The number of known and 

 named species is already as high as this in several dis- 

 tinct classes, as, for instance, in the diatomes of the 

 primitive plants and the radiolaria of the primitive ani- 

 mals. These solitary living unicellulars, or "hermit- 

 cells," may be called monohia. 



Many other protists have abandoned this original 

 solitary life; they follow their social instincts and form 

 communities or colonies of cells (ccFnobia). These are 

 usually formed by the daughter-cells which arise from 

 the cleavage of a mother-cell remaining united after the 

 division, and so on with the succeeding generations 

 which come from their repeated segmentation. The 

 following are the chief forms of these coenobia : 



1. Gelatinous Ccenobia. — The social cells secrete a 

 structureless mass of jelly, and remain associated in 

 the common gelatinous mass, without actual contact. 

 Sometimes they are regularly, at other times irregular- 

 ly, distributed in it. We find coenobia of this kind even 

 among the monera, such as the zoogloea of many bacteria 

 and chromacea. They are common among the proto- 

 phyta and protozoa. 



2. Spherical Ccenobia. — The cell-community forms 

 a sort of ball, the cells lying close together at its surface, 

 touching each other or even forming a continuous layer ; 

 such are holosphcura and volvox among the protophyta, 

 magosphcera and synura among the protozoa. The lat- 

 ter are particularly interesting because they resemble 

 the blastiila, an important embryological stage of the 

 metazoa, of which the simple, epithelial cell-layer at 



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