Flatworms

Honors Biology- Spring 2009Honors Biology- Spring 2009

Phylum Platyhelminthes

Soft, flattened bodies

Bilateral symmetry with cephalization

Form and Function

3 germ layers, but acoelomate Without a coelem

Fluid filled body cavity lined with mesoderm

Digestive cavity (lined with endoderm) is only body cavity

Some free-living, some parasitic Specialized tissues and organs

Feeding

Gastrovascular cavity, single opening (serves as both mouth and anus)

Pharynx- extension from mouth that helps pump food into g.v. cavity

Digestion via diffusion

Respiration, Circulation, Excretion

Because bodies are flat and thin…

Respiration: O2 via diffusion

Circulation: nutrients transported throughout cells via diffusion

Excretion: CO2 and other wastes diffuse out

Response

Ganglia- group of nerve cells that control nervous system--- not complex enough to be a brain

Eyespot- used to detect light intensity in environment

Effective at gathering info. about surroundings

Movement

Cilia on epidermal cells

Muscle cells controlled by nervous system

Reproduction

Sexually Hermaphrodites-

contain both sex organs

Pair joins and exchange sperm

Each will lay fertilized eggs

Asexually Fission- splits in 2

and each half grows

Three Classes of Flatworms1. Class Turbellaria

§ Free-living§ Aquatic (fresh or marine)§ Ex. Planarians

2. Class Trematoda

“Flukes” Parasitic

Infects internal organs

Ex.- Schistosoma mansoni (“Blood Fluke”) Burrows thru exposed skin Carried to blood vessels of small

intestine Mature, reproduce, embryos passed

through feces

Causes Schistosomiasis- Eggs clog

blood vessels and cause tissue decay

3. Cestoda

“Tapeworms” Parasitic Live in intestines

of animals, including humans

No digestive system Simply absorb

host’s food

Head is called a scolex- suckers/hooks to attach to intestinal wall

Segments of body are called proglottids Can break off to

form new tapeworms