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Flatworms
PHYLUM PLATYHELMINTHES
CHARACTERISTICS

Unsegmented worms
Simplest
animals with
Bilateral symmetry
No
specialized circulatory
or respiratory system
(uses diffusion to transport oxygen and nutrients)
Excretes
by diffusion
o Cephalization (sense
organs and nerve cells
gathered into a head region)
o Primitive organ systems with
true organs (digestion,
excretion)
Sexual and asexual
reproduction (hermaphroditic
but don’t self fertilize)
Free-living and parasitic
mobile
FREE-LIVING
FLATWORMS
(AQUATIC) EG. PLANARIA
Planaria – cross-eyed fresh water worm
FREE-LIVING FLATWORMS
PLANARIA
Both
marine and freshwater
Most
are small
(less than cm and few mm thick)
Primitive nervous system allows
for coordination of body
movements
Simple eyes that detect light
changes (respond to stimuli!!)
 Locomotion:
cilia help glide through water,
muscles allow them to twist and turn
 Feeding : Some are carnivores , others are
scavangers.
 Gastrovascular cavity with one opening at the
end of the muscular tube called a
pharynx.(used to suck food in to
gastrovascular cavity)
 Gastrovascular cavity-intestine with many
braches.
 Intestine secrete enzyme to help break down
food.
 Digested food can diffuse from the intestine
into nearby cells.
Reproduction
 Sexual – one worm delivers sperm, while other
receives, egg laid in clusters
 Asexual by fission
PARASITIC FLATWORMS
(FLUKES)
EG.
TAPEWORMS
PARASITIC FLATWORMS
TAPEWORMS!!
Scolex
Proglottids
(sections)
form at
anterior end
Found
inside and outside hosts
If inside cannot afford to grow too
large or it will kill its host.
Tapeworm: Have a scolex (head)
with several suckers and ring hooks
---attach to intestinal wall.
Some can reach up to 30 m in
length (found in a sperm whale)
Human tapeworms can be 7 m
Live
in intestines - Feed on
blood, tissue, fluids and pieces
of cells
Lack : sensory organs,
coordination for mobility,
digestive system
o
Highly developed
reproductive system capable
of producing hundreds and
thousands of egg and sperm.
o
No digestive tract – absorb
food as it passed by
developed
a way to
maximize absorption
of nutrients from their
host
 being
flat
 VERY long
(which increases their
surface area to
maximize the
absorption of nutrients)
Schistosoma mansoni
Life cycle:
Swimming larvae
Burrow into snail
Reproduce asexually
Break out of snail
Swim in water
Bores through skin of
human to blood
vessels and then
intestines
Blood vessels leak
eggs into intestines
Very sick human
Tapeworm life cycle
Mature proglottids break
off posterior
Eggs are released
Animal eats
contamination
Forms a dormant cyst
Human eats
Hatch in host