Probably the most well known flatworm is the tapeworm but there are actually over 20,000
species of these fascinating fellows. The group of flatworms, also known as platyhelminthes,
is typically split up into four different subgroups: turbellaria,
cestoda, trematoda and monogenea though this is debated. Turbellaria are considered the
“free-living” flatworms (which we’ll explain later); cestoda are usually called tapeworms,
trematoda are flukeworms and monogenea… well they’re just monogenea. The free-living flatworms
are flatworms that are able to live independently from other organisms while most other flatworms
are parasitic in nature meaning they require a host organism or host organisms to survive
and complete their lifecycle. It’s guessed that around 80% of flatworm species are parasitic.
Flatworms have bilateral symmetry and it’s been suggested that they may be the oldest
living ancestors to all bilaterally symmetric animals. That includes us since we have bilateral
symmetry. What this means is that if you cut a bilaterally symmetrical organism in half,
both pieces would look the same except reversed. It’s also called mirror symmetry.
While they don’t have a skeleton, a circulatory system or a respiratory system they do have a
nervous system meaning they have a brain and even eye-like receptors called ocelli. They
have different methods of locomotion depending on the species. Some, which you may have guessed,
use their muscles to move their bodies in a wave-like pattern others have tiny little
feet on their undersides called cilia that they use to cruise along slime trails, kind of like
snails! Of course, unlike snails they don’t have a shell as they are all soft-bodied organisms.
They’re also not segmented like some other worms we’ve previously discussed on this show.
Flatworms live both terrestrially and in the water and basically everywhere on earth even Antarctica!
They can come in a variety of colors and some will even appear translucent. Their size range also
varies pretty drastically between the species. On average one could say that they are typically less
than an inch (2.54cm) in length, but really they can reach up to 50 feet (15.24m). I mean you’ve
probably heard some tapeworm horror stories. They also typically only have one opening in their body
that serves multiple purposes meaning they use the same hole both as a means for taking in food
and for excreting it after it has been digested. Here’s to hoping they don’t have tastebuds!
The free-living flatworms are mostly carnivorous and will eat something as small as a single-celled
organism like a protozoan to something much larger than them like crabs and molluscs. The parasitic
ones will feed off their hosts in many different ways, it really just depends on the relationship.
Nearly all flatworms possess male and female reproductive organs so they can usually reproduce
with or without a partner. Some are capable of regenerating from pieces of the original parent.
But really a lot depends on how they live. It’s likely that free-living flatworms typically
produce asexually, or without a partner, but a lot of the parasitic types need to have
certain conditions met in order to continue on in their life cycle. Here’s an example:
some species of tapeworms don’t become sexually mature until they reach a higher body temperature
and this is achieved by ending up in a bird host after living in the cold-bodied fish that the bird
will eat. Unless they make it to that point, they won’t ever mature. So flatworms have even gone
so far as to manipulating their hosts in order to end up where they need to. That cold-bodied fish,
they’ll just make it swim to warmer waters and get fat making them slower and easier targets
for the birds to consume both them and their uninvited guests. It’s all pretty diabolical.
They don’t always win, though, tapeworms eaten by grizzly bears in North America will die
during the bear’s hibernation. Mother nature sure has some intuitive checks and balances.
For more information on flatworms check out the links in the description.
Let us know in the comments if you have an animal to suggest.
Thank you for watching and we’ll see you next time on Animal Fact Files!