GENERAL ZOOLOGICAL INTEREST. 115 



activities, and each cell has usually one function dominant 

 over the others. The Metazoan cells, in acquiring an in- 

 creased power of doing one thing, have lost the Protozoan 

 power of doing many things. 



The Protozoa remain at the level represented by the 

 reproductive cells of higher forms, and are comparable to 

 \ reproductive cells which have not formed bodies. In the 

 sexual colonies of Volvox, however, we see the beginning of 

 that difference between reproductive cells and body cells 

 which has become so characteristic of Metazoa. The 

 Protozoa are self-recuperative, and in normal conditions 

 they are not so liable to " natural death " as are many celled 

 animals. Weismann and others maintain that they are 

 physically immortal. 



They illustrate (a) the beginnings of reproduction, from 

 mere breakage to definite division, either into two as in 

 fission, or in limited time and space into many units, as in 

 the formation of spores within a cyst ; (b) the beginnings of 

 fertilisation, from " the flowing together of exhausted cells " 

 and multiple conjugation to the specialised sexual union of 

 some Infusorians, where two individuals become closely 

 united ; (c) the beginnings of sex, in the difference of size 

 and of constitution sometimes observed between two con- 

 jugating units ; (d} the beginnings of many celled animals 

 in the associated groups or colonies which occur in several 

 of the Protozoan classes. These colonies show a gradation 

 in complexity. Raphidiophrys and other Heliozoa form 

 loose colonies, which arise by the want of separation of the 

 products of fission. Among the Radiolarians, there are several 

 colonial forms, in these the individuals are united by their 

 extra-capsular protoplasm, but are all equivalent. In Pro- 

 teropongia the cells show considerable morphological dis- 

 tinctiveness, some are flagellate, some amoeboid, some 

 encysted and spore forming. Again, in Volvox, as we 

 noticed above, the cells of the colonies show a distinction 

 into nutritive and reproductive units. 



Lastly, in their antithesis of passivity and activity, con- 

 structive and destructive preponderance, anabolism and 

 katabolism, the Protozoa illustrate the phases of the cell 

 cycle, and so furnish a key to the variation of higher animals. 



