PHYLUM PLATYHELMINTHES
"Platyhelminth" means “flat worm”
There are 20.000 species of platyhelminthes
The Phylum Platyhelminth contains four classes
Taxonomy Classification/Representative Members
Class Turbellaria: Worms in the Turbellaria Class are free living and
nonparasitic. They don't need a host to supply their food because they can
hunt smaller animals or eat dead animals.
Class Monogenea and Class Trematoda: Worms in the Monogenea and
Trematoda classes are commonly known as flukes. Both types of flatworms
live on or in other animals.
Class Cestoidea: Worms in the Cestoidea class are commonly known as
tapeworms. Tapeworms are parasitic and live in vertebrates and humans.
They look different than most flat worms because they don't need to digest
their food. They attach to the intestine's lining of their host with their
scolex (head) which has suckers or hooks.
A. Diagnostic Characteristics



Flatworms are simple, unsegmented invertebrates. There are about 100,000
extant species of flatworms and over 50,000 of them are parasitic.
Although flatworms occupy a wide variety of environments, moisture in the
habitat is esential for maintenance of proper osmotic balance (5 NK).
Flatworms only live in marine, freshwater or damp terrestrial habitats.
Flatworms are more complex than cnidarians.
Flatworms are known as “flat” because they are very thin dorsoventrally
(they are thin between their front and their back.) Because they are so thin,
they barely need any organs; each cell can do gas exchange and excrete
waste on its own. However, flatworms are tirploblastic,which means they
possesses three different layers of cells in their bodies. The endoderm is
the innermost layer, the mesoderm is the middle layer that leads to muscle




tissue and the gastrovascular cavity, and ectoderm is the outermost layer
that turns into skin.
The head of flatworms are usually off-set in some way that makes the head
distinguishable from the other parts of a flatworm. On this head are usually
a very simple form of eyes (12 BL).
Flatworms, similar to radiate animals (organisms that resemble a barrel with
equal parts sticking out like the spokes of a wheel), have a gastrovascular
cavity (area of digestion with a way to send the food to the rest of the
body) with one opening. Because flat worms are so dense and do not have any
open cavities (spaces) around their gastrovascular cavity, they are called
acoelomates (a solid organism with no space between the gut and the outer
body.)
The skin of flatworms is soft and ciliated-this means it is covered with
cuticules and with external suckers or hooks(or both for connection to a
host organism) (7 VM).
Flatworms are bilaterally symmetrical organisms, meaning their right and
left sides are mirror images (this also implies that flatworms have a defined
and distingushable top/bottom and head/tail). (8 Nangia)
The anatomy of a typical flatoworm. Notice the lack of a body cavity. (JAC)
http://kentsimmons.uwinnipeg.ca/16cm05/16labman05/lb5pg5_files/flatworm2.jpg
B. Acquiring and Digesting Food
· Digestive is system incomplete- flatworms have a mouth but no anus(7 VM). The
system is typically very branched(7 VM).
On the stomach of a planarian, there is a muscular pharynx (where food and air are
swallowed). When the planarian finds food, stomach juices are sprayed on the prey
spreads out in the worm into three branches that have smaller sub-branches so all
the cells in the worm can be fed. The mouth is at the end of the pharynx that is
far away from the body. It ingests food, and excretes undigested waste.
· Tapeworms have no need for a digestive system. The hooks and and suckers on
the scolex allow the tapeworm to absorbs the digested food from the host's
intestines.
C. Sensing the Environment
· On a planarian's head, there are two eyespots that sense light, and lateral flaps
that allow the flatworm to smell for its prey or other dead animals. The planarian
also uses its nervous system to sense and react to the environment. Planarian’s
nervous system is mostly in the middle of the worm and is more complex than a
cnidarian's nerve net. On its “head” near the eyespots and lateral flaps, there are
ganglia (big groups of nerve cells). The ganglia nerve cords run down the entire
length of the worm’s body. The chemical signals sent through the ganglia down the
nerve cords allows planarians to change their responses to stimuli. Because
planarians have ganglia they can be conditioned and "taught." Research has shown
that planarians are light sensitive. In some planarians light sensitive areas are
arranged in cups, serving as two eyes. (Jesse Carmen) [10]
As a result of the bilateral symmetry, planarians can sense their environment
better. The cephalization or specialization at the front of the organism has been
show to increase mobility, lead to higher order activity, and increase the reactivity
of the organism to its environment. (11 HS)
D. Locomotion
Flatworms usually move through substrates by gliding along with the aid of their
microscopic cilia, minute hairlike organelles, hairs that can be found on the bottom
surface. Some flatworms may move through Peristaltic motion, which consists of
the worm contracting its muscles around the front forcing its anterior end to form
a sharp point. This way of moving is generally slow, but it is very effective and it
does not use up a lot of energy. (14) (BMB) ·Planarians move by using cilia on the
outside layer of their stomachs. They make a thing layer of mucus so they can
move easily with their cilia. Occasionally, they can use their muscles to swim
through large areas of water with an undulating motion (a wave like up-down
motion.)
E. Respiration
Flatworms are so thin that all their cells individually have gas available to them.
Instead of having special organs to help them release and absorb gas, all their cells
do it independently. More specifically, since the flatworm's bodies are so flat,
there is no need for any sort of major respiratory system because there is a small
enough diffusion distance for gas exchange between the environment and all the
organism's cells, so oxygen can be absorbed directly through the organism's outer
body. (6) (SS)
F. Metabolic Waste Removal
· Flatworms metabolic waste is the form of a nitrogen compound (ammonia). The
ammonia leaves the flatworm’s body by diffusing right from the thin body into the
surrounding water. Flatworms also have a simple excretory apparatus that helps
control osmotic balance between the flatworm and its surrounding water.
G. Circulation
· Flatworms don’t have special organs for circulation since they don't need special
ways to respirate. Their nutrients are carried throughout their body by the
gastrovascular cavity. This is known as an open circulatory system. As described
above an open circulatory system is when there is no blood vessils everything is
just in the gastrovascular cavity.(SJB)(8) Once the food is pulled through the
mouth and flows through the pharynx, the nutrients are digested by the cells lining
the inside of the cavity and they are sent through the tubes throughout the body
to feed the worm's cells.
H. Self Protection
· In planarians, the ganglia nerve cords that run down the entire length of the
worm’s body allow planarians to change their responses to stimuli. This nerve cord
runs down the worm's belly instead of its back, a characteristic that separates
them from chordates (OZ). They are able to sense when they need to change
direction in order to stay alive.
I. Osmotic Balance
·Because flatworms are flattened dorsoventrally and only have three layers of
cells, many of the flatworm's cells are touching the water they live in. Water is
simply used directly from their habitat to fuel metabolic processes. Also,
flatworms have a simple excretory apparatus that helps control osmotic balance
between the flatworm and its surrounding water. This apparatus is made of flame
cells (cilia cells). The flame cells move fluid through branches of the gastrovascular
cavity to the outside water. This function allowed some flatworms to evolve in
freshwater and damp land areas.
J. Temperature Balance


Worms in the Turbellaria class are the temperature of their habitat. Their
habitats are limited to water bodies and damp terrestrial habitats because
they have no means to protect themselves if their habitat gets too cold or
too hot.
Thus, flatworms are ectotherms - they lack the ability to regulate their own
internal temperature.(ER)
K. Reproduction
·The reproductive system of each sex is complete with gonads, ducts, and
accessory organs. Fertilization occurs internally. The eggs are microscopic, and the
development in its life cycle is either direct or with one or more larval stages(7
VM).
Planarians can reproduce asexually (without having sex) by regeneration ( regrowth of body parts from pieces of an organism.) The parent breaks itself in two
halves by folding in half length wise and the halves re-grow the end that is missing.
Planarians also can reproduce sexually. One planarian is a male (makes sperm) and a
female (makes eggs), which is known as a hermaphrodite. Sometimes mates cross
ertilize (mate with other species)
Important terms:






Dorsoventral- thin between their front and their back
Tirploblastic- possesses three layers of cells in their embryos
Endoderm- innermost layer
Mesoderm- middle layer that leads to muscle tissue and organ systems
Ectoderm- outermost layer
Radiate- organisms that resemble a barrel with equal parts sticking out like
the spokes of a wheel















Gastrovascular cavity- area of digestion with a way to send the food to the
rest of the body
Acoelomates- a solid organism with no space between the gut and the outer
body
Flame cells- cilia cells
Planarians- The group of flatworms in the group (genus) Dugesia lives in
unpolluted ponds and streams; eat dead animals or hunt smaller animals
Undulating motion- a wave like up-down motion.
Ganglia- big groups of nerve cells.
Pharynx- where food and air are swallowed
Asexually- with out having sex
Regeneration- re-growth of body parts from pieces of an organism.
Hermaphrodite – organism that is a male (makes sperm) and a female (makes
eggs)
Cross fertilize- mate with other species
Flukes- Class Monogenea and Class Termatoda
Host- the organism that houses and feeds the parasite
Scolex- head of a tapeworm
Proglottids- sacs of sex organs that make up the body of a tapeworm