>Phylum Platyhelminthes > Class
Turbellaria > Order Polycladida
updated Feb 14
learn only 3 things about them ...
are fragile and tear easily. Don't touch!
They are fearsome predators and hunt and eat other animals.
really flat has some advantages. Can you think of some?
Marine flatworms are common on all our shores. They range from tiny
ones found under rocks to larger ones that roam out in the open. Some
are brightly coloured and patterned, others blend with their surroundings.
Flatworms are usually more active when it is dark when they busily
slide about with elegant ruffles of their body edges, or even swim.
What are flatworms? Flatworms
are unsegmented worms belonging to the Phylum Platyhelminthes. 'Platyhelminthes'
means 'flat worm'. There are about 18,500 species of flatworms, but
only about 16% of these are free-living flatworms. Most members of
this Phylum are internal parasites. These infest fish and other animals
including humans; such as tapeworms and liver flukes.
Marine flatworms belonging to the Order Polycladida, however, are
not parasites. They are free-living flatworms that earn an honest
living by hunting down and eating other animals.
Sometimes confused with: nudibranchs
and slugs. More on how
to tell them apart.
Features: Most are about 1cm
long or less, although some 'monster' species 8-10cm long are also
commonly seen. There are also countless minute free-living flatworms
that live among sand grains. These flatworms stick onto the sand grains
with paired glands on their underside. One gland secretes a glue,
and the other gland another substance to release the glue.
Flatworms are generally leaf-shaped, some have highly ruffled edges.
Some flatworms have tiny tentacles over their heads. Most of the commonly
seen flatworms only have pseudotentacles on their heads, a pair of
ear-like structures made out of folded edges of their bodies. They
are not real tentacles like those of a snail.
Flat-footed advantage: Unlike
bristleworms and earthworms
which are segmented and belong to Phylum Annelida, flatworms are unsegmented
and really very very flat. Usually less than 1mm thick! Being flat
has its advantages. They can get into almost every kind of space:
to hide or to get at their food. (see below for how some eat clams).
Oxygen diffuses quickly across the skin and to all parts of the body.
So a flatworm doesn't have a blood circulatory or respiratory system.
Nutrients also quickly diffused from the central gut to the rest of
the body, although larger flatworms may have a highly branched digestive
system to bring food to the furthest reaches of the body. 'Polyclad'
means 'many branches' referring to their branched digestive system.
But being flat means most can't 'swallow' their prey. Instead, the
pharynx (a part of the gut) is pushed out through the mouth. The pharynx
engulfs the prey outside the worm's body. Or digestive juices are
injected into the prey and the resulting liquefied meal is then sucked
up. Most flatworms don't have an anus and they spit out indigestible
bits through the mouth. The mouth of a flatworm is on the underside
of the body, in some, towards the centre or the back end of the body.
The skin of a flatworm is covered with cilia (tiny beating hairs).
The swirling of their constantly beating cilia gives their group name
'Turbellaria' which means 'whirpool'. In bigger flatworms, the cilia
are often only found on the underside. They also produce mucus that
protects them from drying out or perhaps as protection from predators.
Flatworms have a central nervous system and a simple brain to co-ordinate
their well developed muscular system.
Rainbow Worms: Flatworms come
in a bewildering variety of colours and patterns. Some of these colours
are due to the colour of the prey they have eaten, showing through
their gut. Those with bright colours that contrast with their surroundings
probably serve as a warning of their distasteful nature. Flatworms
can contain powerful toxins. Others have colours and patterns that
blend with the background. These and smaller ones are simply overlooked.
Some flatworms mimic other animals such as nudibranchs
(or perhaps visa versa).
What do they eat? Many flatworms
are carnivores that prey on tiny animals (protozoa, copepods, worms)
or feed on immobile animals such as bryozoans,
ascidians and molluscs.
Being flat, they slip easily between the shells of bivalves
and some flatworms are considered pests of oyster farms. Some are
scavengers, feeding on dead animals.
Flatworms on the hunt: Flatworms
are quite adept hunters. Flatworms may capture prey with their bodies
or entangle it with slime. Some produce paralysing mucus and a few
use their penis to stab their prey!
Some have tentacles or pseudotentacles sense their surroundings. Others
have sensory cells to detect water currents and chemicals released
by potential food. A few also have balance sensors that tell them
which way is up. Some have simple eye spots on their head or along
their body margins. These don't form an image and only help flatworms
detect movement and avoid the light.
Flatworms on the move: To move
about, small flatworms secrete a mat of mucus and crawl on this mat
with a dense layer of cilia on their underside. Bigger ones may swim
by undulating the sides of their bodies. Some large flatworms even
have a sucker on their undersides to get a grip on the surface.
What eats them? Among their predators
slugs (Family Aglajidae).
Flatworm babies: Marine flatworms
are hermaphrodites, that is, each flatworm has both male and female
reproductive organs. When two flatworms meet, they exchange sperm.
Some species simply insert their needle-like penis anywhere in the
body of the partner. This is not surprisingly called 'hypodermic impregnation'.
In yet other species, each flatworm tries to impregnate the other
without itself being impregnated, as it involves more energy to produce
eggs. This results in a sort of 'penis-fencing' when the two flatworms
meet! Here's a video clip of 'penis-fencing'
shared by Loh Kok Sheng.
Eggs are laid in gelatinous strings or masses. Some flatworms lay
only a few eggs. Most flatworms do not have a free-swimming larval
stage. Instead, miniature flatworms develop within the protective
egg capsule. Here is a photo
of a flatworm larva on Image
Quest 3-D Marine Library
Fragile worms: Flatworms are
very delicate and tear easily when handled. So please avoid touching
Status and threats: None of our
flatworms are listed among the threatened animals of Singapore. However,
like other creatures of the intertidal zone, flatworms are affected
by human activities such as reclamation and pollution. Trampling by
careless visitors, and overcollection of their food source can also
have an impact on local populations.
Some may be colourfully patterned.
St John's Island, Nov 12
made out folded edges of the body margin. Mouth is on the underside.
Pulau Sekudu, Jul 05
flatworms may undulate the sides
of their bodies to 'swim'.
Pulau Semakau, Jan 05
mouth of a flatworm may be towards
the middle of the underside of the body.
Pulau Hantu, Jan 06
Yellow clustered bead ascidians?
Changi, Jun 08
flatworms are tiny.
This one was hardly 1cm long.
St. John's Island, Mar 05
may be nearly transparent!
Sisters Island, Feb 10
Beting Bronok, May 09
Polycladida recorded for Singapore
Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in
*Ng, Peter K. L. & N. Sivasothi, 1999. A Guide to the Mangroves of
Singapore II (Animal Diversity)
in red are those listed among the threatened
animals of Singapore from Davison, G.W. H. and P. K. L. Ng
and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened
plants and animals of Singapore.
+from our observation and identification based on Newman, Leslie and
Lester Cannon. 2003. Marine Flatworms: The World of Polyclads.
seen awaiting identification
Species are difficult to positively identify without
On this website, they are grouped by external features for convenience
Meixneria furva (DD: Data deficient)
With grateful thanks to Leslie H. Harris of the Natural
History Museum of Los Angeles County for comments on and identifying
some of these flatworms.
- Newman, L.J.
& Cannon, L.R.G. Nine new species of Pseudobiceros (Platyhelminthes:
Polycladida) from Indo-Pacific. Pp. 341-368. in the Raffles
Bulletin of Zoology vol 45.
- Newman, L.J.
& Cannon, L.R.G. Colour pattern variation in the tropical flatworm, Pseudoceros (Platyhelminthes: Polycladida), with descriptions
of three new species. Pp. 435-446. in the Raffles
Bulletin of Zoology vol 43.
- Newman, L.J.
& Cannon, L.R.G. Pseudoceros
(Platyhelminthes: Polycladida) from the Indo-Pacific with twelve
new species from Australia and Papua New Guinea. Pp. 293-323.
in the Raffles
Bulletin of Zoology vol 46.
G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore
Red Data Book: Threatened plants and animals of Singapore.
Nature Society (Singapore). 285 pp.
- Newman, Leslie
and Lester Cannon. 2003. Marine
Flatworms: The World of Polyclads.
CSIRO Publishing. 97pp.
- Humann, Paul
and Ned Deloach. 2010. Reef
Creature Identification: Tropical Pacific New World Publications.
- Kuiter, Rudie
H and Helmut Debelius. 2009. World
Atlas of Marine Fauna
. IKAN-Unterwasserachiv. 723pp.
Terrence M., David W. Behrens and Gary C. Williams. 1996. Coral
Reef Animals of the Indo-Pacific: Animal life from Africa to Hawaii
exclusive of the vertebrates
Sea Challengers. 314pp.
- Allen, Gerald
R and Roger Steene. 2002. Indo-Pacific
Coral Reef Field Guide.
Tropical Reef Research. 378pp.
- Edward E.
Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate
Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
Jan A., 2005. Biology
of the Invertebrates.
5th edition. McGraw-Hill Book Co., Singapore. 578 pp.