Download 27.1 Flatworms : Phylum Platyhelminthes

27.1 Flatworms : Phylum Platyhelminthes
(to remember: “Platy” means “Flaty”)
Soft, flattened worms that have tissues and internal organ systems.
Ex) planaria, tubellarians, flukes, tapeworm
First animals to have: three germ layers
bilateral symmetry
cephalization
Recap:
Coleom
Body Plan
Flatworms are acoloemates, meaning they don’t have a true body cavity.
-Body cavity lined with tissue made
from mesoderm.
- No coelom forms between the
tissues of flatworms
-Flatworms have bilateral symmetry
Two well-formed sides that can be
Identified as left and right
-Cephalized: have a distinct head
Feeding
Some are free-living, some are parasitic
To eat, flatworms have a muscular tube called a pharynx, that they extend out of
the mouth, and use as a pump, to move food into their digestive cavity. Once
digested food diffuses to the rest of the body tissue. Undigested food is ejected
from the pharynx. (one opening—no anus)
http://www.youtube.com/watch?v=A6N8KZqGAJg
Planaria eat a worm segment, observation of pharynx
Parasitic worms live on the tissue of other animals (suck blood and nutrients);
tapeworms, blood flukes
Excretion
Flatworms have flame cells that remove excess water and cellular waste (such as
ammonia) from the body. Flame cells join together to form an internal network of
tubes that empty through pores on the worms’ skin.
Respiration and Circulation
Flatworms are thin, and most of the cells are close to the outside—they get
oxygen by diffusion. They have no respiratory system or circulatory system.
Movement
Free-living flatworms usually move by using cilia to glide through water, and
muscle cells, which are controlled by their nervous system to twist and turn in
response to stimuli.
Parasitic worms usually require a host to be moved.
Response
Most flatworms have a concentration of nerve cells called ganglia at their head.
They are not a true brain, however, they control the nerve cords that run along
both sides of the body.
Many free-living worms have eyespots, that detect changes in light. Free-living
worms are more complex and use their nervous system to find food and hiding
spots. (In lab, you will see that planaria avoid light)
Reproduction
Most free living flatworms are hermaphrodites, meaning they have both testes
and ovaries. During sexual reproduction, two worms join and exchange sperm.
Eggs are laid after fertilization.
Can also reproduce asexually by splitting in two (called fission).
Planaria are able to regenerate their tissue and for two new identical planaria.
Video explaining stem cells and regeneration part 1: http://www.youtube.com/watch?v=J9EuFuJF9N0
Part 2: http://www.youtube.com/watch?v=YBLDFI4OhWY
http://www.youtube.com/watch?v=8Ew3yupNMF8&feature=related : Planaria with 8 heads
Turbellarians – Free-living flatworms that live in fresh and salt water.
Flukes – parasitic flatworms that infect internal organs of their host. They infect
multiple hosts. Blood flukes reproduce sexually in humans by living in the
intestine. They release embryos into fecal matter, which go on to contaminate
another host.
Tapeworms—have no digestive tract—they eat food that has already been
digested by their host. Their sperm fertilizes their own eggs, or the eggs of
another, the proglottids break off and are passed out in fecal matter.