Download Phylum Platyhelminthes

Platyhelminthes
PLATY (FLAT) HELMINTHES (WORMS)
Habitat:
y Flatworms can be found in all habitats: Freshwater,
marine, and terrestrial. They may be free-living or
commensals (one benefits and the other is not hurt).
Structure:
y The simplest animals with bilateral symmetry,
exhibit cephalization, have three distinct cellular
layers (ectoderm, mesoderm and endoderm), have
an organ-system level of construction. Digestive
system is incomplete (no anus). Usually only a few
millimeters thick.
Ingesting food:
y Free-living forms have a muscular tube called a
pharynx that they use to suck food into their
gastrovascular cavity. Parasitic forms usually
absorb digested food from their host directly through
their body wall.
Digesting food:
y Both extracellular and intracellular digestion takes
place as in cnidarians. Mechanical digestion occurs
from the suctorial action of the pharynx, and the n
chemical digestion occurs within the incomplete
digestive system and within the cells of the
endoderm.
Elimination:
y No anus: Therefore undigested food is eliminated
through the mouth as in the cnidarians.
Internal Transport:
y No specialized circulatory system. Because they are
so flat they can depend on diffusion to transport
oxygen and nutrients to their tissues.
Respiration:
y No specialized respiratory system. Oxygen diffuses
into the flatworm’s body and carbon dioxide diffuses
out.
Excretion:
y Freshwater forms have a simple excretory system
consisting of a series of tubules. Flame bulbs draw
water out of the tissues and into the tubules. Tufts of
cilia in the tubules create currents – excess water
exits via excretory pores. Primary function of the
system is to rid the flatworm of excess water; some
waste products are probably removed as well. This is
similar to the contractile vacuoles found in certain
protists.
Locomotion:
y Many muscles arranged around the body allow for
complex movements: Longitudinal muscles, circular
and dorso-ventral muscles. Muscular waves extend
down the body during movement. Also, cilia on the
bottom (ventral) surface of the flatworm aid in
locomotion (minor role). Mucus is also secreted
along the ventral surface.
Sensing Environment:
y Can detect touch, water currents, and chemicals with
specialized sensory cells projecting from their body
surface. Many flatworms have light sensitive organs
called ocelli, or eyespots
Coordination:
y Have a centralized nervous system with a definite
head end where a simple brain is located. (Parasitic
flatworms have a very reduced nervous system as
they have no need for complex coordination and
sensing of the environment.
Reproduction:
y Either sexual or asexual reproduction occurs in free-
living flatworms. Parasitic flatworms only reproduce
sexually with very complex life cycles. Most
flatworms are hermaphrodites – they have both
male and female organs. Hermaphrodites join in
pairs and exchange sperm with each other.
Planaria
have a
remarkable
ability to
regenerate.
You cut the
flat worm in
three, you
get three
new
worms…
Class Turbellaria:
y Free living flatworms such as the planarians and the
y
y
y
y
marine flatworms
About 3,000 species.
Mostly carnivores that prey on small animals or
scavengers on larger ones.
Most are less than 5mm long and best seen under the
microscope.
Majority of species are marine
Class Trematoda:
y Over 6,000 species of parasitic flatworms.
y Usually parasites of mollusks, but some (the flukes)
infect humans.
y Some flukes are external parasites, but most are
internal, and infect the blood and the organs Eg.
Chinese liver fluke, sheep liver fluke, blood fluke.
Class Trematoda:
y Blood flukes are found primarily in tropical areas and
certain species infect the blood vessels of the human
intestines. Humans, therefore, are the primary host, of
the host where adult parasites are found and in which
sexual reproduction of the parasite occurs.
y Produce 10,000 to 100,000 times more eggs than free-
living flatworms!
y Humans infected by blood flukes get very sick and can
die. Second biggest world health problem next to
malaria.
Class Cestoda:
y Parasitic tapeworms
y About 3,500 species.
y Adults are almost all intestinal parasites of vertebrates.
y Range from 1mm in length to over 30 meters (in sperm
whales)
y No mouth or digestive cavity at any stage of development
– nutrients from the host are simply absorbed directly
through the tapeworms very thins body wall (tegument
– natural outer covering).
Class Cestoda:
y Nervous system is much less developed than in free-
living flatworms.
y
y Adult attaches to the intestinal wall with suckers and/or
hooks on the scolex.
y
y Body is divided into repeating units (proglottids), each
with one or more complete reproductive system.
y
y Have a mainly anaerobic metabolism as an adaptation to
low levels of oxygen in their hosts intestines.