The third stage in our ancestral series is the turbellarian.  This is a little, flat, oval worm, varying greatly in size in different species, and found both in fresh and salt water.  Some would deny that this worm belonged in our series at all.  But, while doubtless considerably modified, it has still retained many characteristics almost certainly possessed by our primitive bilateral ancestor.  The different parts of hydra were arranged like those of most flowers, around one main vertical axis; it was thus radiate in structure, having neither front nor rear, right nor left side.  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.  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.