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Bell Pettigrew Museum
of Natural History
Interpretative Panels
Text: Dr Iain Matthews
Design: Steve Smart & Cavan Convery
A University of St Andrews Development Fund Project
School of Biology
http://biology.st-andrews.ac.uk
5:2 Platyhelminthes
P hy l u m
Body Plan:
• Bilaterally symmetrical
• Coelomic body cavity with
through gut or blind gut
• Triploblastic
• Acoelomate, with blindending gut
Platyhelminthes
S u b p hy l u m
Super class
Class
Gut
Endoderm
Mesoderm
Ectoderm
The 25,000 species of Platyhelminths are bilaterally
symmetrical, solid-bodied animals with true guts,
although the gut is blind ending with the mouth also
serving as the anus. They are mainly distinguished from
other bilateral phyla by what they lack, rather than any
special unifying feature.
regarded as a primitive character, although some
zoologists argue that the flatworms are descended from
animals with a coelom. Either way there are no flatworms
that show any sign of a coelom, and the space between
the organs and the body wall is filled with irregular
shaped cells forming a parenchyma.
This group can claim to be the closest to the ancestral
body form, that is they are the Bilateria that lack a body
cavity, lack a blood system, lack a through gut and lack
any distinguishing features. The lack of a circulatory
system to distribute respiratory gases has required the
flatworms to adopt the one positive feature they share,
their flatness. Thickness of the body must be kept small
in at least one plane (the dorsoventral one) to allow
sufficient diffusion to meet the oxygen demands (and
remove CO2) from the solid body. Larger forms must
therefore be ribbon or leaf-like in their design. Food
has also to be distributed throughout the body and the
gut must also therefore be highly branched with
diverticulae extending throughout the body.
The classification of Platyhelminthes has traditionally
split the phylum into four classes; the free-living
turbellarians (e.g. Geoplana variagata, ), the monogenean
flukes (e.g. Epibdella hippoglossi,
), the trematode flukes
(e.g. Fascioloides magna,
) and the gutless tapeworms
or cestodes (e.g. Echinococcus multiocularis,
). However
recent molecular analysis has revised the taxonomy to
reveal five classes, all of which are dominated by
turbellarians. The previous three parasitic classes are now
all contained in a single class, Neoophora.
Most species of flatworm do possess ‘excretory organs’
in the form of protonephridia, although again diffusion
is probably the main process and the main function of
the protonephridia is osmoregulatory: these cells are
especially developed in freshwater species. Despite the
adaptations to larger body size there are still constraints
on diffusion and most species remain microscopic.
Most Bilateria possess some form of body cavity, however
the flatworms are solid (acoelomate). This is usually
Classification
within
Platyhelminthes
Class: Acoelomorpha
Order: Nemertodermatida
Order: Acoela
Class: Catenulidea
Order: Catenulida
Class: Macrostomomorpha
Order:Macrostomida
Order: Haplopharyngida
Class: Polycladidea
Order: Polycladida
Class: Neoophora
Order: Lecithoepitheliata
Order: Prolecithophora
Order: Proseriata
Order: Tricladida
Order: Kalyptorhynchida
Order: Typhloplanida
Order: Temnocephalida
Order: Dalyelliida
Order: Aspidogastrea
Order: Digenea
Order: Monogenea
Order: Cestoda
See specimen.
Flatworms, flukes and tapeworms
The tapeworm, (Monieza expansa,
) is a commercially
important parasite of cattle. Platyhelminths have no
circulatory system to move oxygen around the body and
many species have become long (up to 6 m) and flattened
to allow gas to diffuse into the body.
The eggs of Monieza expansa are passed out in the
faeces of the host cattle and are eaten by the intermediate
host, a soil mite. The eggs must be eaten with 24 hours
or they dry out and die.
Many platyhelminths are parasitic, often with complex
multi-host lifecycles. For example, the broadfish
tapeworm (Diphyllobothrium latum,
) has two
intermediate hosts, a copepod and a fish. The definitive
host is a fish-eating vertebrate such as a bear, a seal or
a human. In humans, adult tapeworms can reach lengths
of 10 metres (>30 feet) and produce over a million eggs
a day.