Chapter 13 - Introduction
to Phylum Platyhelminthes
General Characteristics
• Bilaterally symmetrical; dorsoventrally flattened
• Acoelomates; most of the body is comprised of parenchyma, a
collection of fibers and cell types (sensory, food storage, etc.); internal
organs are embedded in the parenchyma; muscle fibers are also found
running through the parenchyma
• Unable to synthesize fatty acids and sterols de novo
• Turbellaria and
free-living stages
(certain larval stages)
of Class Cestoidea
and Trematoda have
ciliated epithelium
General Characteristics cont.
• The nervous system does
have a centralized
(ganglionated) area and a
peripheral system (arranged
as a network or as a
ladderlike system with
longitudinal nerve cords and
transverse commissures)
• The digestive system is
typically a blind sac and most
flatworms have a mouth near
the anterior end followed by a
muscular pumping pharynx
General Characteristics cont.
•Anus absent; wastes are eliminated through the mouth or via
nephridiopores
• In some the gut is a simple sac, while in others it is finely branched
structure; some groups (cestodes) the gut is lacking
• Osmoregulatory
system is comprised of
flame cells or
protonephridia; water
and waste exit the body
through pores called
nephridiopores
General Characteristics cont.
•Most species are monoecious,
with cross fertilization
• In some turbellarians and
cestodes groups sperm transfer
is by hypodermic
impregnation
Platyhelminths Systematics
Traditional Classification Scheme
1. Class Turbellaria: Most free-living worms from terrestrial, FW and
marine environments
2. Class Monogenea: All parasitic, mainly ectoparasites of fishes (e.g.
found on the skin and gills)
3. Class Trematoda: All parasitic, mainly in the digestive tract of
vertebrates
a. Subclass Digenea - at least a 2 host life cycle; first host is
a mollusc
b. Subclass Aspidogastrea - most with only one host
(mollusc)
c. Didymozoidea - tissue dwelling parasites of fish
4. Class Cestoidea: All parasitic in all classes of vertebrates except
agnathans; most have an intermediate host
Characteristics of Revised Classification Scheme
• The subphylum
Catenulida is a sister
group of the “true”
Platyhelminthes
• The main structural
feature separating the
subphylum Catenulida
from the subphylum
Euplatyhelminthes is
the presence of a frontal
organ in all the latter;
Euplatyhelminthes also
possess dense dermal
ciliature compared to
catenulids
Revised Classification Scheme cont.
• Among the S.P.
Euplatyhelminthes, the
acoelomate forms
(Superclass
Acoelomorpha) lack a
protonephridia and
generally do not have a
gut
• The Superclasses
Rhabditophora have
some kind of
protonephtridia
Taxonomic groups that will be emphasized
Superclass Rhabditophora
Class Trematoda
Subclass
Aspidobothrea
Subclass Digenea
Class Monogenea
Class Cestoidea
Subclass Cestodaria
Subclass Eucestoda