BEGINNINGS ILLUSTRATED BY PROTOZOA 



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various activities, and each cell has usually one function 

 dominant over the others. The Metazoan cells, in acquir- 

 ing an increased 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 



Fio. 71. — A colonial flagellate Infusorian — Proterospongia 

 haeckelii. — After Saville Kent. 



There are about 40 flagellate individuals, a, Nucleus ; b, contractile 

 vacuole ; c, amoeboid unit in gelatinous matrix ; d, division 

 of an amoeboid unit ; e, flagellate units with collars contracted ; 

 /, hyaline outer membranes ; g, spore-formation. 



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 m.ere 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, Heliozoa, Sporozoa, etc. — where two 

 individuals become closely united ; along with this, the 



