GENERAL INTEREST OF PROTOZOA. 113 



of the Protozoon series, in which the dilTerentiation of parts in a unicel- 

 lular animal reaches its highest point " (Lankester). 



Examples. — Acineta, suctorial ; Dendrosoma, forming branched 

 colonies, suctorial ; Ophryodendron, non-suctorial. 



General Interest of {"rotozoa. 



The Protozoa illustrate, in free and single life, forms 

 and functions like those of the cells which compose the 

 many-celled animals. 



They remain at the level represented by the reproductive 

 cells of higher forms, and are comparable to reproductive cells 

 which have not formed bodies. They 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, or in 

 limited time and space into many units ; {V) the beginnings 

 of fertilisation, from " the flowing together of exhausted cells " 

 and multiple conjugation to the specialised sexual union of 

 some Infusorians ; \c) the beginnings of sex, in the difference 

 of size sometimes observed between two conjugating units ; 

 (d) the beginnings of many-celled animals, in the associated 

 groups or colonies which occur in several of the Protozoon 

 classes. 



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

 structive preponderance and destructive preponderance, 

 anabolism and katabolism, they furnish a key to the variation 

 of higher animals. 



