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 pre-digested 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

NecessaryOxygen 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 necessaryOne worm breaks into small pieces as a form of reproductionEach piece creates a new worm

PlanariansMost 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 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

TapewormsHave 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