Traditional classifications divide the
Platyhelminthes into four groups: Turbellaria and the wholly
parasitic Trematoda,
Monogenea and
Cestoda. and vulnerable to fluid loss. The
nervous system is concentrated at the head end.
Diet and digestion Most other turbellarians are carnivorous, either preying on small invertebrates or
protozoans, or scavenging on dead animals. A few feed on larger animals, including
oysters and
barnacles, while some, such as
Bdelloura, are
commensal on the gills of
horseshoe crabs. These turbellarians usually have an eversible pharynx, in other words, one that can be extended by being turned inside-out, and the mouths of different species can be anywhere along the underside. Most turbellarians have pigment-cup
ocelli ("little eyes"), one pair in most species, but two or even three pairs in some. A few large species have many eyes in clusters over the brain, mounted on tentacles, or spaced uniformly round the edge of the body. The ocelli can only distinguish the direction from which light is coming and enable the animals to avoid it. In turbellarians there are one or more pairs of both
testes and
ovaries. Sperm ducts run from the testes, through bulb-like seminal vesicles, to the muscular penis. In many species, this basic plan is considerably complicated by the addition of accessory glands or other structures. The penis lies inside a cavity, and can be everted through an opening on the posterior underside of the animal. It often, although not always, possesses a sharp
stylet. Unusually among animals, in most species, the sperm cells have two
tails, rather than one. In most species "miniature adults" emerge when the eggs hatch, but a few large species produce
plankton-like
larvae. ==Taxonomy and evolution==