Phylum: Platyhelminthes - The Flatworms I Chapter 10 I F.Sc. I First Year I biology I Lec. 12

General Characteristics:
Name.
The name platyhelminthes means “flatworms”. The body of these animals is soft and
dorsoventrally compressed.
Triploblastic organization.
The Platyhelminthes are triploblastic acoelomates. There is development of a third
layer, the mesoderm, which separates the ectoderm and endoderm.
Symmetry.
The Platyhelminthes exhibit bilateral symmetry, and body is unsegmented.
Lifestyle.
As a parasites.
With few exceptions the Platyhelminthes are parasites, mostly endoparasites,
i.e, live inside their hosts. The most common examples are
 Taenia solium (Tapeworm),
 Fasciola hepatica (Liver fluke) and
 Schistosoma (blood fluke).
The parasites are more common in tropics. Some of these cause diseases in humnas.
As a free living.
A few species are free living and found in freshwater, for example Dugesia (planaria).
Size
Their size ranges from few millimeters (10 mm in case of Planaria) to several meters
(tapeworm).
Digestive system.
Much of the body space is taken up by a branching sac type digestive system.
The digestive system is poorly developed in some species (parasitic) or may be absent
as in the tape-worms.
Excretory system
The excretory system consists of branching tubes ending in bulb-like cells, the flame
cells.
Nervous system.
A well-developed nervous system is present in platyhelminthes. It is in the form of
either a simple network of nerves or ganglia.
Sense Organ.
The sense organs are present at the anterior a simple network of nerves or ganglia.
The sense organs are present at the anterior end.
Systems Absent.
Respiratory and circulatory systems are absent.
Mode of Nutrition.
The parasitic species absorb nutrients from the hosts. The free-living species (Planaria)
feed on small animals and bodies of dead and decaying animals

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