Hello marine biology students.
In this video we're going to talk about a
diversity of bilaterally symmetrical worms.
[Intro Music]
There are many phyla which fall into this
category of bilaterally symmetrical worms.
The first of these that we'll talk about are
the flat worms, phylum Platyhelminthes.
When we look at the basic characteristics
of these worms, they have bilateral symmetry,
so a top and a bottom, and a right and a left
side.
They have three tissue layers and they have
a Central Nervous System
including a brain, which is a cluster of nerve
tissue in the head.
They have a gastrovascular cavity like we
saw with the cnidarians, in that it was a
single opening to that digestive compartment,
that one opening functions as both the mouth
and an anus.
They do not have a body cavity to store their
internal organ, but they do have organs embedded
within their body wall.
When we look at the basic lifestyle of these
flatworms, several are free-living flatworms,
which we call turbellans, some are going to
be parasitic flukes and some will be tapeworms.
When we look at the reproduction of these
flatworms, asexual reproduction can happen
through fragmentation 
and regeneration.
Sexual reproduction is also possible.
These flatworms are hermaphroditic having
both male and female reproductive organs.
Here we see these two turbellan flatworms
participating in a mating ritual known as
penis fencing.
In this case, both flatworm have a needle-like
penis and they are attempting to inject the
other.
For the flatworm that is successful, they
will deposit sperm into the body of the other
and they avoid the cost of both healing the
wounds and developing eggs that would be fertilized
by those sperm cells.
Our next phyla of bilaterally symmetrical
worm are the ribbon worms from phylum Nemertea.
What causes the nemerteans to be distinct
is that they have this long proboscis that
they can use to surround and ensnare their
prey items.
They are all carnivores and they feed on other
worms.
So their basic characteristics, they have
a long elastic body with a proboscis.
This proboscis, it can evert from a pocket
right above their mouth and it can surround
their prey items.
Nemerteans have a complete digestive tract,
meaning it's a digestive tube containing both
a mouth and an anus at distinct ends of that
digestive tube.
There are other advancements in their bodily
anatomy as well.
They have a body cavity, which the flat worms
did not, even though it is somewhat reduced
and they have a circulatory system.
Some of these ribbon worms can grow incredibly
long, with one species reaching up to 30 meters
or 100 feet in length, making it one of the
longest invertebrates known.
There are tapeworms that are found in whales
that can also measure more than 100 feet in
length and even some of the tentacles of the
cnidarians which we had talked about earlier
can reach lengths in excess of 100 feet, so
even though an organism is an invertebrate
they can still be exceptionally long.
Our next phylum of worms are the nematodes,
phylum Nematoda.
Most of these nematodes are very small, although
some parasitic varieties can be large enough
to see and even dissect as needed.
They are bilaterally symmetrical with a complete
digestive system and they have something called
a hydrostatic skeleton, in that there's an
internal fluid-filled compartment that the
muscles of the body can compress against to
allow for movement.
Nematodes often live in sediments in the marine
environments and salt marshes and even in
other terrestrial environments as well.
And they can range from being just a member
of the sediment community, eating detritus,
to potentially being parasites.
Estimates on the actual numbers of species
in this phylum range anywhere from 10,000
to 25,000, but it's likely even significantly
higher than that.
They make up such an important component of
sediment communities and just throughout the
world there are so many environments for us
to test and identify all the nematodes that
are present.
One particular nematode of interest come from
the genus Anisakis.
Here it's larvae can be found within fish
and if fish is consumed raw by humans or other
organisms, these parasitic larvae can end
up being introduced to these organisms, causing
a variety of illness and other symptoms.
Our next group of worms are found primarily
within the plankton.
They are fish-like worms known as Arrow worms
or Chaetognaths.
They come from phylum Chaetognatha.
The basic characteristics of chaetognaths
is that they are planktonic
and widely distributed in all oceans.
They have transparent streamlined bodies with
fish-like fins and a tail.
They are voracious predators in their environment
where they can swim rapidly and feed on small
planktonic crustaceans, fish larvae and other
plankton.
There are about 130 recognized species, all
of which are marine.
The last group of bilaterally symmetrical
worms that we'll discuss are the annelids
or segmented worms.
They are in phylum Annelida.
Their basic characteristics are that they
have internal and external body segmentation.
They have a well-developed nervous system
and brain.
They have a closed circulatory system.
They have bilateral symmetry 
and their body cavity is a true coelom which
is lined with tissues derived from the mesoderm
as in higher animals.
There are several different groups within
phylum Annelida that are of importance in
the marine environment.
The first of these being the polychaetes.
The prefix poly means many and chaetae referred
to bristles.
And so, these polychaetes have many bristles
on the outside of their body.
They are the largest and most diverse group
of segmented worms with about 10,000 species
nearly all of them being marine.
Each segment has a pair of flattened extensions
or parapodia, often with gills for gas exchange
and bristles called setae or chaetae.
Polychaetes range from being free-living carnivores
to deposit feeders, some are tube worms and
others are suspension feeders.
Here we see representatives of some of the
polychaetes, with the major polychaete structure
being shown in the illustrated diagram with
their mouths and eyes and segments, the parapodium
with the bristles or setae and then we see
a few photographs of individuals in the wild.
The first one, labeled (a), this is a fireworm
and those red structures that you see along
its back are its gills.
What is less obvious are the glassy-like needle-like
hairs that are especially painful if you ever
touch them and so I would strongly encourage
you not to touch fire worms because their
name lives up to the sensation that it feels
like.
And then we see some of the tube worms with
their bristles modified into a feeding appendage,
allowing them to collect food particles suspended
in the sea water.
This is why they are suspension feeders.
The next group of segmented worms are the
pogonophorans or beard worms.
And if you thought some of the other marine
worms were weird, these are especially weird.
So, these beard worms have long extensions
or tentacles and it turns out that they do
not have a digestive system at all.
Instead they absorb nutrients through these
tentacles and sometimes they even have symbiotic
bacteria and archaea living with them.
An animal of this size that does not have
a digestive system is really remarkable.
As I had mentioned, many have symbiotic bacteria
and some of these bacteria are even chemosynthetic
and that allows these beard worms to live
in environments such as deep sea hydrothermal
vents 
and methane seeps.
As we continue our discussion of the annelids,
another group are leeches.
While leeches are maybe more abundant in terrestrial
aquatic environments, there are marine leeches
as well.
They have anterior and posterior suckers which
allow them to hold to prey.
They parasitize, invertebrates and fish.
The photo on this slide is showing an interesting
type of worm known as an Echiuran, and what
causes echiurans to be distinct from some
of the other annelids is that externally they
do not have clear segmentation.
They have a non-retractable spoon-shaped or
forked proboscis for feeding.
They are deposit feeders and they often burrow
in soft sediments yet some live in corals.
The fat innkeeper worm in the Pacific Northwest
is an example of an echiuran.
The last of the segmented worms that we'll
talk about are another one that secondarily
lost their segmentation.
These are the sipunculans or peanut worms.
The sipuncs, as they are sometimes called,
have a retractable proboscis
that is used for feeding.
They are found mainly in shallow water.
They are deposit feeders, burrowing in soft
sediments.
That finishes our discussion of worms.
Now, before our next video I would like you
to think about what your favorite type of
seafood is.
Mine is coming up in the next video.
All right, see you then.
