Download Chapter Four: Phylum Platyhelminthes

Chapter Four: Phylum Platyhelminthes
The Flatworms
Who are the Platyhelminthes?
Well, let’s get one thing straight, most of the species that belong to this phylum are not your
friends. There are more than 20,000 species of these guys divided into four classes and most of them are
not very pleasant to have around. Why you may ask? It certainly is not because of their flattened body
shape, but rather because three out of the four classes tend to be parasites living and feeding in
unsuspecting hosts. The fourth class, the turbellarians, are not parasites and live in either fresh or salt
water.
Why do flatworms either have to live in aquatic, moist land environments or as parasites? Now
this actually does depend on their body structure. They are flat because they are missing the coelom, an
open area where the organs are found, so they are are also missing some body parts that we take for
granted. Higher animals, including humans, have a hollow body cavity between the endoderm and the
mesoderm, the coelom, where we store our lungs, heart, digestive system etc. Flatworms do not have this
coelom (making them acoelomate) and thus are missing a circulatory and respiratory system. Since they
have no circulatory or respiratory system, they need to be flatter so the gases can diffuse into and out of
their bodies through their skin.
Flatworm body symmetry and structure:
Okay, we already know that they are acoelomate which makes flatworms flat, but what else do
flatworms have in common? They are a little more complex than the sponges and cnidarians in that they
are bilaterally symmetrical and show some basic signs of cephalization. What? Let’s put this in easier
terms. They have a definite head and rear end and they have a primitive brain. They still are stuck with
the one way gut thing though so what goes in must also go out the same way. Of course, the digestive
system needs a fancier name so we say that flatworms have a single gastrovascular cavity that both
ingests food and excretes any undigested material back out through the “mouth”.
The free living flatworms are the turbellarians and most of them live in the ocean. Some of them
are quite beautiful and can get fairly large, more than a few inches long. However, unless you make a
trek out to the ocean, you are more likely to encounter a planarian here in Milan. You have to know what
you are looking for though. These worms are tiny, maybe half an inch at the max and somewhat
transparent to make it even harder to find them. The planarians have a triangular-shaped head and move
either by swimming in a wavy manner or gliding on a trail of mucus kind of like a snail.
Cerebral ganglia
Gastrovascular Cavity
Planarians eat by taking food in through their pharynx, passing it on to their gastrovascular cavity
which secretes enzymes to break the food down. Planarians also have to have a way to get rid of excess
water so they don’t get bloated which could happen fairly easily since they live in water. They get rid of
extra water by using flame cells. The flame beat little cilia which look like candle flames and cause the
water to move into the flame cell which then can empty it back into the water outside of the body.
How are the parasitic flatworms different from the free living flatworms?
* Flukes
Two of the classes, the trematodes and monogeans are ectoparasites which means they live on the
outsides of their hosts. They are more commonly called flukes and have been known to hang out on
people as well as other animals. The biggest difference between the parasitic flatworms and the freeliving ones are that they need some structure to help them attach to their host. They have an anterior
and ventral sucker to cling on to the host. Basically this looks like a suction cup on the top of their
“head” and under their “chin”. They attach themselves to their host and then start sucking out fluids from
their host to supply their nutrients (kind of like a vampire!). They also are missing eyespots because once
they settle in to feed they don’t really need to move around that much. Additionally they don’t seem to
have as many sensory structures because they don’t need to move around much once they find a spot they
like.
One fluke you may have heard of is a blood fluke belonging to the genus Schistosoma which
causes the “swimmer’s itch” infection. The adult fluke hangs out inside human blood vessels. Humans
are the primary host because that’s where the adults live. Once they mate inside of you, the eggs go to
the intestine or bladder and leave the body, well, you know how, with the urine or feces. If the eggs get
into water, the larva swim until they find their intermediate host, a type of snail. The larva develop in
the snail until they grow tails and make a break for it. Once they get back in the water, they look for a
human with exposed skin and start the cycle over again. My, that sounds like fun.
* Tapeworms
The last class of flatworms is the most delightful if you have a morbid sense of humor. These are
the cestodes or tapeworms. They specialize in living in the intestines of almost every animal out there.
People have been known to carry 7 different kinds of tapeworms. The tapeworms have the simplest
bodies but the most complex life cycles of all the flatworms. They got rid of all the excess body
structures. Who needs eyespots, a mouth, digestive organs or a gastrovascular cavity when you can latch
on to a juicy intestine and feed right through your skin? They do need a more complex hook system to
latch on to the intestine with though. So they developed an organ called a scolex at the anterior end that
has hooks and suckers to cling to the intestine wall. The body is made up of body sections called
proglottids and when it grows, it adds new proglottid sections. Some of these suckers have reached 10
meters in people. Don’t you think you would know if you had a 10 meter (33 foot) long worm growing in
your intestines?
Just like the flukes, tapeworms have a primary host (often us) and an intermediate source. So how
do you get a tapeworm? If you eat undercooked beef or pork (and probably other meat as well) you may
accidentally eat a cyst. What is a cyst? Well, remember the tapeworm body is made of proglottids? The
proglottids can break off and be excreted in your feces where they can stay alive for several months. If a
cow or pig eats something with the proglottid on it, the proglottid enters its body and releases eggs. These
eggs develop into larvae that make their way to muscle tissue (what we eat) and form cysts which are
dormant larva surrounded by a protective coating. If you do not cook the meat long enough, you don’t
kill the worms so they can emerge and find a new home in you. Way fun.
hooks
Proglottids
Sucker
Scolex
Neck
Close up of a Tapeworm
Photo Credits:
Planaria: http://www.bergen.cc.nj.us/Faculty/rdill/Environmental_Bio/Lab_Images/planaria.jpg
Labeled Planaria: http://www.usd.edu/biol/labs/151/PlanariaWM.htm
Flame cell: http://homepage.smc.edu/hodson_kent/Animal/ANIMKG.HTM#flame
Tapeworm closeup: http://www.parasitology.co.uk/