Bilateral symmetry
Exhibit cephalization
Sac body plan acoelomates

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.

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

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)

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!

Glide over the bottom of the water with cilia
Twist and turn to move away from stimuli using muscles

Asexual – by fission
Common among free-living flatworms

Most free-living flatworms are hermaphrodites
The worms join and exchange sperm
Eggs are laid in clusters- hatch in a few weeks

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

Most familiar free-living flatworm

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 6-
10 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

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