flagella, the colony is broken up, the different individuals settle
to the bottom, and each gives rise by division to a new colony. This
group of cells may be considered as a colony or as an individual.
Each term is defensible.

Volvox is also a spheroidal organism, composed often of a very large
number of flagellated cells. But it differs from magosphæra in
certain important respects. In the first place its cells have
chlorophyl, the green coloring matter of plants. It lives therefore
on unorganized fluid nourishment, carbon dioxide, nitrates, etc. It
is a plant. But certain characteristics render it probable that it
once lived on solid food and was therefore an animal. For where
almost the sole difference between plants and animals is in the
fluid or solid character of their food, a change from the one form
into the other is not as difficult or improbable as one might
naturally think. And plants and animals are here so near together,
and travelling by roads so nearly parallel, that, even if volvox
never was an animal, it might still serve very well to illustrate a
stage through which animals must have passed.

The cells of volvox do not form a solid mass, but have arranged
themselves in a single layer on the outer surface of the sphere. For
a time, under favorable circumstances, volvox reproduces very much
like magosphæra, and each cell can give rise to a new, many-celled
individual. But after a time, especially under unfavorable
circumstances, a new mode of reproduction appears. Certain cells
withdraw from the outer layer into the interior of the colony. Here
they are nourished by the other cells and develop into true
reproductive elements, eggs and spermatozoa. Fertilization, that is,
the union of egg and spermatozoon, or mainly of their nuclei, takes