Download Ch 26 - Platyhelminthes

Ch 26 Platyhelminthes
Flatworms
Platyhelminthes
Bilateral symmetry
Exhibit cephalization
Sac body plan
acoelomates
Acoelomate
Free-living,
aquatic
flatworms eat
plankton or
recently dead
animals
They use the
pharynx to suck
the food into the
gastrovascular
cavity
The protruding
pharynx is visible, as
the worm feeds on
the daphnia. The
mouth is at the end
of this extension. The
pharynx will pin down
the prey while
enzymes secreted
from the mouth
soften the tissue. The
mouth sucks in the
food and digestion is
completed inside the
cells.
Planaria Feeding
Structure
Digestion begins in the gastrovascular cavity
Smaller particles are absorbed by the walls of
the cavity (the intestinal walls) where digestion
is completed
The gastrovascular cavity branches throughout
the body, so nutrients are absorbed directly
from the intestinal cells
Wastes are expelled through the mouth
Parasitic flatworms
feed on blood,r
tissue fluids or predigested food
found inside the
host’s intestines
Some have hooks
or suckers instead
of mouths, and
they simply absorb
the nutrients
through their
epidermis
No Circulatory or
Respiratory Systems
Necessary
Oxygen diffuses directly into their flat bodies
Carbon dioxide and metabolic wastes diffuse out
directly
Fresh-water flatworms have flame cells to get rid
of excess water (as contractile vacuoles did in
amoeba)
More Developed
Nervous Systems
Have a “brain” in
their head
Nervous system
runs throughout
body
Two Ventral nerve
cords run down
sides of body
Free-living
flatworms have
ocelli to detect light
Chemoreceptors
test water for food
Motion detectors
are scattered
around the body
Parasitic flatworms do not have developed nervous
systems
No need!
Locomotion
Glide over the
bottom of the
water with cilia
Twist and turn to
move away from
stimuli using
muscles
Reproduction
Asexual – by fission
Common among
free-living flatworms
Sexual
Most free-living flatworms are hermaphrodites
The worms join and exchange sperm
Eggs are laid in clusters- hatch in a few weeks
Regeneration
Most flatworms are
able to regenerate
structures if
necessary
One worm breaks
into small pieces as
a form of
reproduction
Each piece creates
a new worm
Planarians
Most familiar free-living flatworm
Blood
Flukes
Parasitic flatworm
Have two different
hosts in their life
cycle
Found in tropical
areas
Most flukes are
hermaphroditic
Produce huge
numbers of eggs
Cause the intestinal
cells to burst,
leaking blood and
eggs into the
intestine
The male is about 610 mm
Female is longer
and thinner and
lives in the groove
which runs the
length of the male’s
body
Eggs are passed out with the feces
Where there are poor sewer systems, the eggs get
into the water system
Swimming larvae hatch from the eggs
Swimming larvae find their intermediate host – a
snail
They reproduce asexually in the snail and digest
it’s tissues
The new worms swim until they find a human
They bore into the skin, eat their way to the blood
vessels and travel to the intestines, making the human
very ill
Blood flukes often kill their human host
Have a scolex with
several suckers and
a ring of hooks
Attach to intestinal
walls
Absorb nutrients
through their body
walls
Can be up to 18
meters long
Tapeworms
Constantly produce
proglottids
Contain male and
female reproductive
organs
Proglottids rupture
full of eggs, or break
off
Eggs and proglottids
are passed out with
the feces
Food or water contaminated with eggs is ingested
by an intermediate host (cow, pig, fish)
Larvae from eggs burrow into muscle tissue of host
and create a cyst
Eating uncooked meat containing the cysts will
begin the cycle again