foreign particles from the lungs. The turbellaria is still covered
with cilia, probably an inheritance from the gastræa; for, while in
smaller forms they may still be the principal means of locomotion,
in larger ones the muscles are beginning to assume this function and
the animal moves by writhing. The bilateral symmetry has arisen in
connection with this mode of locomotion and is thus a mark of
important progress.

In the turbellaria we find for the first time a true body-wall
distinct from underlying organs. The outer layer of this is a
ciliated epithelium or layer of cells. Under this an elastic
membrane may occur. Then come true body muscles, running
transversely, longitudinally and dorso-ventrally. Between the
external transverse and the internal longitudinal layers we often
find two muscular layers whose fibres run diagonally. The body is
well provided with muscles, but their arrangement is still far from
economical or effective.

Within the body-wall is the parenchym. This is a spongy mass of
connectile tissue in which the other organs are embedded. The mouth
lies in the middle, or near the front of the ventral surface. The
intestine varies in form, but is provided with its own layers of
longitudinal and transverse muscles, and usually has paired pouches
extending out from it into the body parenchym. These seem to
distribute the dissolved nutriment; hence the whole cavity is still
often called a gastro-vascular cavity as serving both digestion and
circulation. There is no anal opening, but indigestible material is
still cast out through the mouth.

The animal can gain sufficient oxygen to supply its muscles and