New Zealand flatworm,
Arthurdendyus triangulatus - Turbellaria
An invasive species in Europe that feeds almost exclusively
Lancet liver fluke, Dicrocoelium dendriticum
A parasitic fluke of cattle,sheep and other
grazing animals, its intermediary hosts are a land snail and an
ant, rarely it may also infect humans. Infected ants at dusk climb
up a blade of grass to the tip where they hang by their jaws, this
makes them much more likely to be eaten by the next host, the grazing
vertebrate, if they don't get eaten, they join the rest of the ants
for the rest of the day.
Taenia solium, the pork tapeworm -
- scolex with four suckers and two rows of hooks with which
it attaches to the intestine wall of its vertebrate host.
Taenia solium, the pork tapeworm-
proglottids, these develop away from
the scolex maturing as they move closer to the end when they are
then shed with their eggs.
Adult male schistosome or blood fluke
- Tremotoda, the cause of schistosomiasis or bilharzia, the
second most common tropical infectious disease after malaria.
The white bar at bottom left is 0.5mm long. There are estimated
to be around 230 million people in 77 countries who are infected
by one of the six known species.
Blue Pseudoceros Flatworm,
- Turbellaria. While many
flatworms are pretty yucky or at best weirdly interesting,
some of the free living marine Turbellaria are really quite pretty!
picture used permission of Stephen Childs
under Creative Commons Attribution 2.0 Generic License
Another free living marine flatworm,
picture used permission of Nhobgood
under Creative Commons Attribution-Share Alike 3.0 Unported License
- Bilateral symmetry (single line
of symmetry, down the middle - like you)
- Soft bodied, no skeleton
- Simple nervous system with a rudimentary
brain, some have ocelli, simple eyes that detect
the direction of light and dark
- No internal body cavity
- Blind gut, mouth but no anus,
most waste passes out the way it came in
- No circulatory or respiratory system
- Diffusion for respiration, restricts
the animals to a thin, flattened body shape
- Excretory system for metabolic waste
consists of a branching network of flame cells, the protonephridial
Kingdom - Animalia
Phylum - Platyhelminthes
The group includes:
- Cestoda - Tapeworms.
Most adults are endoparasites of vertebrates and are highly
adapted to this role. There is usually a primary host and also
one or two intermediary hosts which is often an invertebrate.
They have no gut and absorb nutrients directly from the
digestive tract of the host where they live. They have a surface
that itself is increased in surface area to absorb nutrients
just like the gut wall of their host.
Typically a tapeworm
has a head or scolex which attaches itself and then
a series of proglottids that are reproductive segments
(they may be mistaken for Annelids in this regards as they look
superficially segmented in the same manner). The proglottids
grow towards the end of the worm here they break off to release
eggs with the faeces of the host.
- a genus that includes economically
important livestock parasites such as those of pigs and
cattle. It is thought that such parasites have a part to
play in pork being a "forbidden" meat in some
made up of Flukes, parasites which usually have two
hosts, one of which is a vertebrate, the other being an invertebrate,
often a snail. There is a mouth towards the front of the animal
and a sucker towards the tail end by which they hold onto their
host. They are usually small animals from around 0.2 to 7mm
- Liver flukes
- parasites of a wide range of vertebrates
such as sheep, deer, camels and man.
- Lung and intestinal flukes
- many members of this group infect
the gut or connected structures of the primary host. Lungs,
bile ducts, pancreatic ducts and intestines are common sites.
- the cause of Shistosomiasis or Bilharzia, the second commonest
and debilitating tropical disease after malaria.
- Turbellaria - The
most primitive group of flat worms. Most members are free-living
and marine, though a few species are parasitic. Generally small
in size, less than 5mm with some being microscopic, the largest
grows up to 60cm. They are frequently brightly coloured often
being camouflaged if not.
Individuals are hermaphrodite
(male and female at the same time, some larger marine species
have an unusual mating behaviour called penis fencing!
The loser is impregnated and takes the more energetically
expensive female role of growing the eggs which hatch into planktonic
They move by cilia on the under-surface of the
body (ventral surface).
a group that are well known for the ability to regenerate
if cut in half or even several small pieces including growing
a new head if the piece doesn't have one.
What do flatworms eat?
the most highly specialized of the flatworms. All are endoparasites
(internal parasites). There is no digestive tract, they absorb nutrients
directly across the body wall from the digestive tracts of the vertebrates
that are their primary hosts. They are usually caught by eating
raw or uncooked contaminated fish, beef or pork.
Trematodes are almost all endoparasites.
They feed on the cells, cell fragments, tissue fluids or blood of
their host. They are one of the most widespread and serious groups
of human parasites.
They usually have a vertebrate as the primary host and then one
or maybe two intermediary hosts. The Chinese liver fluke for example
has man as its primary host and then a snail and a fish as its intermediary
hosts. The parasite produces cercaria larvae released by
the snail and then burrow through the skin of a fish where they
encyst in the muscle, they are then eaten by man where they migrate
to the liver where they make eggs that are released in the faeces
to be taken in by snails.
Some Trematodes such as Schistosomes are very highly
adapted parasites, rarely causing death, despite being one of the
greatest plagues in tropical regions causing weakness and suffering
for many years in the human hosts. In the intermediary snail hosts,
Schistosomes reach a perfect balance as a parasite. They
allow the snail to live and feed but not to grow or breed, any excess
food the snail takes in is diverted to the production of parasite
larvae to infect humans the next step in their life cycle. Keeping
the host alive and producing new parasites is a much better strategy
than killing the host when the parasites would almost always
die as well.
Turbellarians are predators on smaller animals,
invertebrates or protozoans and scavengers.
One species known as the New Zealand Flatworm is an
invasive species in Europe where it arrived in the 1960's. It eats
earthworms so resulting in a reduction of the soil quality.
One species called Symsagittifera roscoffensis
(previously Convoluta roscoffensis) a small 15mm long marine
species found along the Atlantic coast of Europe swallows algal
cells which live as endosymbionts, it doesn't have to feed at all
as an adult. In some places the bright green colouration resulting
has earned it the name "mint sauce worm".
What eats flatworms?
Free living flatworms, the Turbellarians
are soft bodied and slow moving, they have species that live
in fresh and sea water this makes them ready prey for a whole host
of fish, crustaceans, amphibia, diving beetles, insect larvae that
can make a meal of a small worm.
There don't seem to be any animals that are specialists in eating
flatworms, probably because they are easy to catch when found and
frequently hide so can't be relied on as a prey animal, instead
being part of the diet of many generalists and opportunists.
As Cestodes and Trematodes are overwhelmingly parasitic, they
are the top of their foodchain. Parasites are either eaten but not
digested by their next host in the case of eggs and dormant larvae
or not eaten at all.
Some gill flukes of fish may be eaten by cleaner wrasse on coral
reefs, these small fish help clean up larger fish even swimming
into their mouths and gills to do so, the larger fish benefit from
the association and so don't try to eat the small fish that they
could do easily.