The protozoa include also the flagellata, a great, very poorly
defined mass of forms occupying the boundary between the plant and
animal kingdoms. They are usually unicellular, and their protoplasm
is surrounded by a thin, structureless membrane. This prevents their
putting out pseudopodia as organs of motion. Instead of these they
have at one end of the ovoid or pear-shaped body a long,
whiplash-like process or thread, a flagellum, and by swinging this
they propel themselves through the water. These flagellata seem to
have a rather marked tendency to form colonies. The first individual
gives rise to others by division. But the division is not complete;
the new individuals remain connected by the undivided rear end of
the body. And such a colony may come to contain a large number of
individuals.
[Illustration: 2. MAGOSPHAERA PLANULA. LANG, FROM HAECKEL.]
Such a colony is represented by magosphaera. This is a microscopic
globular form, discovered by Professor Haeckel on the coast of
Norway. It consists of a large number of conical or pear-shaped
individual cells, whose apices are turned toward the centre of the
sphere. The cells are cemented together by a mucilaginous substance.
Around their exposed larger ends, which form the surface of the
sphere, are rows of flagella, by whose united action the colony
rolls through the water.
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