00:00:01
the phylum platyhelminthes are sometimes
00:00:04
simply referred to as the flatworms
00:00:07
this phylum consists of four different
00:00:10
classes three of which are entirely
00:00:14
parasitic so class turbularia
00:00:18
this is the only free living group there
00:00:21
are a handful
00:00:22
of commensal and parasitic species of
00:00:25
other invertebrates that do
00:00:26
fall in this class but the bulk of
00:00:30
organisms in class
00:00:31
turbularia are not parasitic they are
00:00:34
free living all of the other classes are
00:00:38
exclusively parasitic and they're
00:00:40
actually to believe to have evolved from
00:00:43
one turbularian ancestor
00:00:46
so we have class trauma tota
00:00:50
class sestota and class monogenea
00:00:57
so there is a common homology that all
00:01:00
of the parasitic
00:01:01
platyhelminthe classes share with one
00:01:04
another
00:01:04
and that is they have a living
00:01:07
non-ciliated
00:01:09
sensational tegument now the term
00:01:11
synthesial
00:01:12
basically means that there is there are
00:01:15
no plasma membranes
00:01:17
separating individual cells from one
00:01:19
another so basically
00:01:21
cells all fuse together to form one
00:01:23
large
00:01:24
massive tissue layer basically
00:01:27
that is not cellular so it's
00:01:30
multi-nucleated
00:01:31
and it's all fused into one this
00:01:34
the tegument covers the body so it is
00:01:38
the outermost
00:01:39
layer of cells covering the body
00:01:42
it might secrete fibrous layers
00:01:46
external to it as a matter of fact it
00:01:48
does in certain organisms
00:01:52
but regardless the cellular tegument or
00:01:56
the outermost layer of cells
00:01:59
no longer exists as a layer of
00:02:02
individual cells but
00:02:04
cells that have fused together to form a
00:02:06
syncytium
00:02:07
and this homology is found in all of
00:02:09
them
00:02:10
and this is one of the primary bits of
00:02:12
morphological evidence
00:02:14
for all of these and parasitic
00:02:17
lineages having evolved from one
00:02:20
parasitic
00:02:22
or pre-parasitic ancestor
00:02:25
it also suggests that parasitism arose
00:02:28
only
00:02:29
once to lead to all of these groups
00:02:32
so we have the planaria which is a free
00:02:35
living type of flatworm
00:02:37
and somewhere along the evolutionary
00:02:39
trajectory
00:02:41
some turbularian evolved parasitism
00:02:45
and from that ancestral flatworm that
00:02:48
was once free living that evolved
00:02:50
parasitism
00:02:51
we have the trauma tota uh the the
00:02:54
monogenes
00:02:56
and the sestota all evolving
00:03:01
so the platyhelminthes uh and and i'm
00:03:04
just going to use the basic
00:03:07
turbularian body form there are many
00:03:10
uh adjustments to this that we'll talk
00:03:13
about when we get into each of the
00:03:14
different classes
00:03:15
but the basic body form or the
00:03:17
traditional body form of a platy helmet
00:03:20
is that the mouth is located
00:03:23
somewhere at the anterior end
00:03:27
or at the middle of the body but
00:03:30
always on the ventral surface so these
00:03:32
are worms that are dorsal ventrally
00:03:34
flattened
00:03:36
the mouth leads to a blind digestive
00:03:39
tract
00:03:39
which means that this is not a one-way
00:03:43
street so food doesn't come in travel
00:03:46
down the digestive system
00:03:48
and then leave through a different
00:03:49
opening that would be a one-way street
00:03:51
the digestive system in the plasti
00:03:53
helmets is actually a
00:03:55
two-way street so the digestive system
00:03:58
is blind which means
00:03:59
it's like hitting a dead end so food
00:04:02
enters into the mouth travels through
00:04:05
the digestive system
00:04:06
and then waste material is actually
00:04:08
released back
00:04:09
out through the mouth also there is no
00:04:12
anus
00:04:14
metabolically produced waste is released
00:04:18
across the body wall but
00:04:22
even though this is the normal release
00:04:24
mechanism for nitrogenous wastes
00:04:27
many platyhelminthes particularly the
00:04:29
free living plathe helminthes
00:04:31
do have a very simple excretory system
00:04:35
that excretory system can contains
00:04:39
uh what we call proto-nephritia and
00:04:42
the proto-nephridia are basically
00:04:44
collections of
00:04:45
these little cells they're called flame
00:04:47
cells uh
00:04:48
the fancy word is solenocyte and inside
00:04:52
of the flame cell so what we're looking
00:04:53
at on the left-hand side here
00:04:55
is a planaria body and the entire
00:04:59
system of proto-nephritia is sketched in
00:05:02
this drawing
00:05:03
so we can see there are a number of
00:05:05
small little excretory canals
00:05:08
there's excretory pore that's
00:05:10
highlighted here
00:05:11
but these excretory canals all flow into
00:05:14
a larger excretory canal that'll
00:05:16
eventually channel to the excretory pore
00:05:18
but where is it actually getting the
00:05:21
metabolic waste from well it's being
00:05:22
collected at the tips
00:05:24
so right here right here
00:05:26
[Music]
00:05:27
right there right there at these tips
00:05:30
we're gonna blow that up
00:05:32
come over to the right this is what
00:05:33
we're looking at here this is a
00:05:35
flame cell and you can see the flame
00:05:38
cell
00:05:39
has these closely spaced little rods
00:05:42
that form basically a filtration system
00:05:46
on the surface so the flame cells are
00:05:48
filtering
00:05:49
out essentially the the bodily fluids
00:05:54
and removing nitrogenous waste and then
00:05:57
it's traveling down the excretory canals
00:06:00
eventually to the duct where it leaves
00:06:02
the body at the excretory
00:06:04
pore so it's a very simple excretory
00:06:07
system
00:06:08
uh it's not isolated in one area of the
00:06:10
body you basically have these flame
00:06:12
cells spread throughout the body where
00:06:13
they're collecting
00:06:15
uh nitrogenous waste and waste material
00:06:17
from the cells
00:06:20
almost all flatworms are simultaneous
00:06:24
hermaphrodites
00:06:25
most of them cannot self-fertilize a few
00:06:29
can
00:06:29
but most cannot self-fertilize which
00:06:32
means although they may be simultaneous
00:06:33
hermaphrodites they still need to find a
00:06:35
mate
00:06:36
in order to go through a reproductive a
00:06:38
sexual reproductive event
00:06:40
many parasitic species have complex life
00:06:43
cycles utilizing different forms
00:06:45
in different hosts with several
00:06:48
different larval stages then
00:06:51
they also possess incredible powers of
00:06:54
regeneration
00:06:55
so injuries to the body cuts in the body
00:06:59
can heal something that you know we may
00:07:03
think of in a human as being deadly like
00:07:05
splitting your head in half
00:07:07
is not deadly to many of the
00:07:10
flatworms if the head splits in half
00:07:13
each half grows another half and you now
00:07:16
have two headed
00:07:17
planaria or flatworm
00:07:22
flatworms are bilaterally symmetrical so
00:07:24
they do displace cephalization which is
00:07:26
an isolation of
00:07:29
nerve sensory organs at the anterior end
00:07:32
at one end that we refer to as the
00:07:34
anterior end
00:07:36
um they are all acelamate so they don't
00:07:39
have a body cavity
00:07:40
and they are all triple blastic they all
00:07:42
have an endoderm and ectoderm
00:07:44
and a mesoderm filling the
00:07:48
acylimate area or that internal
00:07:51
area of the body remember there is no
00:07:53
body cavity
00:07:54
but there is a parenchyma parenchyma
00:07:57
tissue is
00:07:58
essentially the soft
00:08:01
internal tissue of the body
00:08:06
so here we're looking at the cross
00:08:07
section of a free living
00:08:10
flatworm uh you'll notice the epidermis
00:08:13
one cell layer thick here uh
00:08:16
this is you can see these are cellular
00:08:18
so this is not showing the tegument
00:08:20
which would actually be sitting just
00:08:21
outside
00:08:22
of this just beneath
00:08:25
here you have a circular muscle layer in
00:08:28
addition to that circular muscle layer
00:08:30
which goes around
00:08:31
the whole body of the flatworm you also
00:08:34
have these
00:08:35
vertical they look like columns these
00:08:37
are the dorso ventral muscles
00:08:40
so contracting would flatten the worm
00:08:42
out more relaxing would allow the worms
00:08:44
body to expand a little bit like it is
00:08:46
here
00:08:47
but regardless of whether those muscles
00:08:49
are completely relaxed or contracted the
00:08:51
worm is going to appear
00:08:52
dorsal ventrally flattened the gut uh
00:08:56
is highlighted in the center here so
00:08:57
this is the lumen of the gut and then
00:08:59
you have
00:09:03
endodermally derived gastrodermis lining
00:09:06
the gut tissue and then all in between
00:09:08
here
00:09:09
it looks like little teeny tiny dots
00:09:11
this is
00:09:13
representing the parenchymal tissue
00:09:19
free living species usually have
00:09:22
well-developed sensory systems
00:09:25
including eye spots as well as
00:09:29
relatively large
00:09:32
nerve cell bodies and neuroglia
00:09:36
in that anterior head
00:09:39
region parasites generally have less
00:09:42
elaborate systems
00:09:45
that are often completely lacking
00:09:48
in any type of of sensory uh
00:09:51
isolation at the anterior end and in
00:09:53
some cases very few
00:09:56
traditional sensory organs like eye
00:09:58
spots that can sense light and dark and
00:10:00
shadows
00:10:01
but more or less they're sensing their
00:10:04
external environment over the length of
00:10:06
their body
00:10:08
through chemoreceptors on their cells
00:10:13
so why would that be well
00:10:16
we're talking about parasites and what
00:10:18
is a common trend that we see in
00:10:20
parasite evolution
00:10:21
when one organism is living in or on
00:10:25
another organism
00:10:26
they do tend to lose some of their
00:10:28
characteristics that they don't require
00:10:31
it's a typical life history trade-off
00:10:34
organisms
00:10:35
do not invest energy where they don't
00:10:37
have to
00:10:38
so if an organism is living entirely in
00:10:40
the gut of a host
00:10:42
what would be the purpose of developing
00:10:45
sensory organs that allow them to sense
00:10:47
light and dark there is no purpose
00:10:50
uh so what we see is the reduction of
00:10:53
the nervous system
00:10:54
in the parasitic species compared to the
00:10:56
free living species
00:10:57
that really need to sense changes in
00:11:00
their environment and whose environments
00:11:02
are much
00:11:03
less consistent than an internal
00:11:05
parasite
00:11:08
so class turbularia again this includes
00:11:12
pretty much all free living flatworms
00:11:14
again handful
00:11:16
of parasites there's about 4 500 species
00:11:20
most are free living most are very tiny
00:11:22
and inconspicuous
00:11:23
many live in marine systems and fresh
00:11:26
water systems there are actually a few
00:11:28
terrestrial species that live in very
00:11:30
moist soils
00:11:32
they move utilizing their muscles both
00:11:35
their circular and their circular
00:11:37
muscles that go around the body and
00:11:38
those dorsal ventral muscles that span
00:11:41
from the dorsal body wall to the ventral
00:11:44
body wall some also have cilia
00:11:48
on their ventral side again this is a
00:11:52
trait that we would see
00:11:53
primarily in free living turbularian not
00:11:56
parasitic the cilia and the mucus that's
00:11:59
secreted along with the cilia helps in
00:12:01
attachment and movement also
00:12:03
position of the mouth is normally around
00:12:05
mid-body
00:12:06
cephalization is present with clear
00:12:09
ganglia neuroglia nerve cords and eye
00:12:11
spots
00:12:12
and the excretory system as well
00:12:14
developed proto-nephritia with flame
00:12:16
cells
00:12:17
throughout the body
