The phylum Platyhelminthes ) is a large and diverse one. We
think usually of flatworms as being planarians, the familiar
little freshwater beasts so common in biology demonstrations,
but the planarians (Class Turbellaria, Order Tricladida) are
members of only one of the four Classes in the phylum. Various
Turbellarians may be free-living or parasitic, and inhabit both
fresh and salt water; the other three Classes are all
exclusively parasitic. Flatworms, so various among themselves,
share certain characteristics. First, they all have a definite
head end, usually with simple eyes, a definite tail end, and are
bilaterally symmetrical. And they are, as the name implies,
flat. Most are small, on the order of 1 to 3 centimeters, but a
polyclad (Class Turbellaria, Order Polycladida) may be 25 or
more centimeters long, and a tapeworm (Class Cestoidea, Order
Cestoda) may be many yards long. The digestive cavity, if there
is one, has only one opening, the mouth, indigestible particles
must leave by the same route. The