The Class Turbellaria contains mostly free-living forms ranging in size from a
few mm to 50 cm. Most species are bottom dwellers in marine and freshwater
environments that crawl over rocks, sand or vegetation. Smaller forms can swim
by means of ventral cilia, but more often they move by laying down a sheet of
mucus that aids in adhesion and helps the cilia gain traction. Larger forms use
powerful muscle contractions to crawl or swim. Unique to turbellarians are
rod-shaped rhabdites found among the ventral epidermal cells of the body
surface. These rhabdites secrete mucus that coats the animal's body, possibly
for protection against predators or to prevent drying.
In terms of nutrition, most turbellarians are predators and scavengers.
Epidermal mucous secretions trap and kill prey items. A muscular pharynx everted
though the ventral mouth is used to secrete digestive enzymes into prey, which
is then sucked into the branched intestine that forms a gastrovascular cavity.
In addition to a simple nervous system, turbellarians have light-sensitive eye
spots called ocelli that help orient the animal to the direction of light. Touch
and chemical receptors in some forms like the planarian seen in lab are
concentrated in lateral projections from the head called auricles that look like
ear lobes. Reproduction in turbellarians can occur asexually through fission or
sexually; all forms are monoecious but practice cross-fertilization. Planarians
are also known for their tremendous powers of regeneration, and a planarian that
has been cut into three pieces will give rise to three new complete individuals!