122 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. 
free and single life, forms and functions like those of the 
cells which compose the many-celled animals. T 'ypically, 
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; (4) 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 ; (¢) the beginnings of sex, in the difference of size 
and of constitution sometimes observed between two con- 
jugating units (e.g. in Coccidium); (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 /voterospongia the cells show 
