Section II. CLASS TREMATODA
Flukes which parasitize humans have been found in most of the systems and
organs of the body. Flukes use more than one host, alternating asexual generations in
one or more hosts (intermediate hosts) with the sexual generation in another host
(definitive host). The flukes are monozoic (one body part), leaf-shaped organisms that
have two heavily muscled suckers. The oral sucker is used as a mouth while the
ventral sucker, acetabulum, is used as an organ of attachment to the host. The
digestive system is incomplete with internal and external digestion. The nervous
system is well developed. Some anatomical structures (for example, the shape and length of
the intestinal ceca, the size and location of the acetabulum, hooklets around the oral
sucker, and the location, the shape, and number of testes) are used for taxonomical
The members of the class Trematoda are commonly known as flukes. The class
is divided into three subclasses: Monogenea, Aspidobothria, and Digenea. The
subclass Monogenea is characterized by having only one host, and its members are
ectoparasites of poikilothermic vertebrates (such as fish, amphibians, and reptiles).
Members of the subclass Aspidobothria are characterized by a relatively simple life
cycle without asexual generations, and are ectoparasites of fish, turtles, and mollusks.
The third subclass, Digenea, is the largest of the three, and encompasses all
trematodes which are parasitic to man. Some members of this subclass are parasites
of many other animals. The subclass Digenea is further subdivided into two orders:
Strigeatoidea which contains the schistosomes, and Prosostomatea which contains the
other parasitic flukes.