This video is about flatworms, like this beautiful triclad flatworm Bipalium,
which I filmed on campus at Cal State Long Beach.
Just take a moment and enjoy that shiny mucus coat.
The animals we’ll look at fall into four clades – triclads, polyclads, digeneans, and cestodes,
and those last two groups being exclusively parasitic.
Highlighting the groups we will look at makes clear that we’re ignoring a ton of flatworm diversity.
Most of these other groups are really only known by flatworm specialists;
the ones we’ll cover are the large free-living ones, or the parasitic ones, that most people are likely to encounter.
So let’s start with triclads.
You’ve all seen these before in the form of “planarians”, which is a common name, not a scientific name.
This is what most people think of when they think of a flatworm.
Here is one very darkly pigmented species.
And here is another species, which we can see more structures on since it’s not quite so opaque.
I didn’t include a scale here, but these animals are about 7 or 8 millimeters in length.
This individual has started crawling upside down on the lid of this petri dish, but then retreats.
And now it’s committed to being upside down for a little while.
This individual is also upside down, so we’re viewing it from the ventral surface,
and that allows us to see some slightly different things than from a dorsal view.
Even though the brown planarian was less opaque than the black species,
it’s still hard to make out details of internal anatomy in a live specimen.
Here’s a way to see the digestive system – feed a flatworm food labeled with some opaque black material,
then kill it and “clear” the tissue – change its refractive index so that it’s more similar to that of the medium it’s in.
That’s what’s been done here,
and this animal has also been stained faintly green.
You can see the three main branches of the gut that give these the name “triclad”.
We can also look at these in cross-section.
Here are sections taken at three different points along the anterior-posterior axis.
Some flatworms are well known for their ability to regenerate missing body parts,
and to heal wounds, as highlighted by this website.
Several species are now model systems for understanding stem cells and regeneration.
To demonstrate this you need to cut flatworms, of course.
So I anaesthetized an individual of this species,
then bisected the anterior end down to about halfway down the body, as shown by this red line.
Wound healing and regeneration were very rapid.
Four days later that animal had two apparently complete heads.
Another clade of flatworms is Polycladida.
That name just refers to the fact that they have many branches of the gut, more than triclads.
These tend to be relatively large in body size.
Here are some local polyclads, the first two from the intertidal zone on the Palos Verde Peninsula.
These are pretty common under cobbles on the shore.
This last species was collected unintentionally by divers at White Point;
they brought back some small stones and it was on one of those.
The cestodes, monogeneans, and digeneans are all parasites,
part of a well-defined clade called Neodermata.
Let’s start with a local digenean.
These are also known as digenetic flukes, or sometimes just flukes.
They have life cycles with at least two hosts.
Locally we can easily find digeneans in an intermediate host, the horn snail Cerithideopsis californica.
Most of the large snails at this site are infected.
We can look at all of the digenean stages in a snail intermediate host by looking directly
at the tissue way up in the spiral of the shell, where the gonad and digestive gland are found.
This snail did not seem to be infected by digeneans…
but this snail was infected.
You can see that the gonad seems completely absent,
or at least obscured by a thick coat of tan or yellow tissue.
If you look more closely at that tissue you see it’s mostly composed of parasite stages.
We can look at those with a compound microscope.
This cercaria failed to get out in the 45 minutes or so that I watched it, but these ones were more successful.
There are at least 19 species of digeneans that use
California horn snails as an intermediate host.
This is one of the three species whose cercariae have a forked tail.
Based on a 2019 paper by Ryan Hechinger, I think this is Mesostephanus appendiculatus, but I’m not completely sure.
It’s hard to obtain live adult digeneans since they live in the digestive systems of birds or other vertebrates.
But we have some prepared specimens of a digenean whose adults
live in the livers of humans and other vertebrates that eat fishes.
This is Clonorchis sinensis.
This is the only adult flatworm we’ll see where we can
get a good look at the complexity of the reproductive system.
We have relatively easy access to live cestodes, or tapeworms, in southern California.
A very abundant elasmobranch in our waters, the round ray, is the final host for numerous species of cestode.
Dr. Ralph Appy has worked out the life cycle of one of these,
and he was kind enough to supply me with live adults, and video of stages that were more difficult to get.
So here is the life cycle of this species.
Adults live in the ray, and they shed eggs into the intestine.
Those exit with feces.
Some of those get eaten by a copepod, where they hatch into procercoid larvae.
That copepod might get eaten by a small fish…
and if it does, those procercoids migrate to the gall bladder and form plerocercoid larvae.
The ray eats the small fish, and the plerocercoids develop into adults again.
Dr. Appy exposed this copepod, Tigriopus, to eggs of Rhinebothrium,
and those eggs developed successfully in to procercoids.
It seems unlikely that Tigriopus is really the natural first intermediate host,
since it lives in a very different habitat than do rays.
But since the procercoids develop successfully in them,
that suggests that at least that the first intermediate host is a copepod.
Plerocercoids have this sucker structure at the anterior end that adults do not have.
That was Rhinebothrium, but Dr. Appy also gave me that larger tapeworm that we saw in the spiral valve,
and that’s a species of the genus Acanthobothrium.
The “acantho” part of that name means spiny, and you can see why that term is appropriate
when you look more closely at the anterior end of the bothria, which we’ll do in a moment.
Ok – one more cestode, just to see a different life cycle.
So this is the dog tapeworm, Taenia pisiformis.
Here’s my rendition of the life cycle, featuring box-shaped mammals.
By “releasing eggs in feces”, I mean that adult tapeworms release eggs into the intestine,
and they’re shed to the outside in the dog feces.
In red are the four distinct stages of the life cycle.
We have two of those to look at, the adult and the cysticercus.

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