ger 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
nerves, which are the principal seats of combustion, through the
external surface. It has, therefore, no special respiratory organs.
But the waste matter of the muscles cannot escape so easily, for
these are becoming deeper seated. Hence we find an excretory system
consisting of two tubes with many branches in the parenchym, and
discharging at the rear end of the body. This again is a sign that
the muscles are becoming more important, for the excretory system is
needed mainly to remove their waste. These tubes maybe only greatly
enlarged glands of the skin.

[Illustration: 5. TURBELLARIAN. LANG.
_va_ and _ha_, front and rear branches of gastro-vascular cavity;
_ph_, pharynx. The dark oval with fine branches represents the
nervous system.]

The nervous system consists of a plexus of fibres and cells, the
cells originating impulses and the fibres conveying them. But this
much was present in hydra also. Here the front end of the body goes
foremost and is continually coming in contact with new con