Section I. OVERVIEW OF PLATYHELMINTHES
The organisms that belong to this phylum show a dorsoventral flattening that
gives them the common name of flat worms. The internal structures are organized in a
bilateral symmetry. The body may consist of only one part (monozoic) or of two or more
parts (polyzoic). Because flat worms are adapted to parasitism, the necessity for
certain functions is no longer needed. Therefore, the organs involved in those functions
were eliminated. On the other hand, with the new demands of parasitism, some of the
organs have adapted to enhance their capabilities.
Skeletal, circulatory, and respiratory structures are lacking within these
organisms. They have no body cavity (acoelomates) and the internal organs are
embedded in a cellular matrix called the parenchyma. The digestive tract may be either
incomplete or totally absent. The nervous system varies from a very primitive network
found in the free living forms, to a well developed arrangement present in the parasitic
forms, that is composed of a pair of anterior ganglia each having a longitudinal nerve
cord. The excretory system varies with each species. There may be flame cells, long
ducts with exterior exit, or bladders with excretory pores.
With a few exceptions (for example, schistosomes), the phylum is characterized by a
monoecious arrangement of the reproductive systems, in which both male and female
reproductive organs are present within a single individual worm. Fertilization is internal
and the exchange of spermatozoa between individuals or between two segments is the
preferred method of fertilization.
There are three classes in the phylum. The class Turbellaria consists of free
living organisms (for example, genus Planaria), while the classes Trematoda and Cestoda
contain all parasitic species.