Flatworms Honors 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: O 2 via diffusion  Circulation: nutrients transported throughout cells via diffusion  Excretion: CO 2 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 Flatworms 1.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