But our little
turbellaria, while still without a head, has one end which goes
first and can be called the front end. The upper or dorsal surface
is usually more colored with pigment cells than the lower or ventral
surface, on which is the mouth. It has also a right and left side.
It is thus bilateral.
The gastraea swam by cilia, little eyelash-like processes which urge
the animal forward like a myriad of microscopic oars. In our bodies
they are sometimes used to keep up a current, _e.g._, to remove
foreign particles from the lungs. The turbellaria is still covered
with cilia, probably an inheritance from the gastraea; 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.
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