Metazoa (animals) > Bilateria > Lophotrochozoa
Platyhelminthes (Greek platy = flat, helminthos
= a worm) are the least complex of the triploblastic animals. Their
characteristic features are a flattened, bilaterally symmetrical,
triploblastic, acoelomate, body form and a simple nervous system.
Though they have a third layer of cells, and a new body layout,
flatworms are acoelomate. This means that they have no cavity
(coelom) in the body between any of the cell layers. Consequently,
organs are surrounded by body cells, limiting their independence of
movement. The similarities and differences exhibited by these groups
suggest that flatworms and ribbon worms represent a grade of
increasing complexity between the Cnidaria and the “higher” animals.
Of the three major classes, members of the
Turbellaria are free-living and often have a simple larva. Like the
larvae of many other marine forms, that of the turbellarians is said
to be planktotrophic, in that it feeds in the plankton. members of
the other two major classes, the Trematoda (flukes) and Cestoda
(tapeworms), are always parasitic, showing a number of adaptations
to a parasitic life-style.