122 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



free and single life, forms and functions like those of the 

 cells which compose the many-celled animals. Typically, 

 they show great structural or morphological simplicity, but 

 great physiological complexity. Within its single cell the 

 Protozoon discharges all the usual functions, while in a 

 higher animal distinct sets of cells have been specialised for 

 various activities, and each cell has usually one function 

 dominant over the others. The Metazoan cells, in acquiring 

 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 

 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 ; (fr) 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 

 beginnings of maturation, as shown in the formation of 

 polar nuclei in some Heliozoa, Sporozoa, Flagellata, and 

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

 and of constitution sometimes observed between two con- 

 jugating units (e.g. in Cocddium)\ (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 Proterospongia the cells show 



