Platyhelminthes (unsegmented) Platy = flat Helminth = worm

General Characteristics Simplest animals with bilateral symmetry Some cephalization (anterior end development) Three germ layers = triploblastic Acoelomate = no mesodermal lined cavity Body has one opening – the mouth

Essential Life Functions Feeding: carnivores or detritus feeders or parasites Symmetry: Bilateral Movement: Motile (aquatic and free-living, parasitic) Response: simple brain, nerve cords, ocelli. Sensitive to chemicals ( most cases, exception tapeworm) Reproduction: asexual + sexual Internal Transport: diffusion, flame cells (planarian) Respiration: diffusion Excretion: expel undigested food thru the mouth

Feeding Free-living – pharynx is on underside of flatworm and sucks food into gastrovascular cavity   intestine branches length of flatworm so digested food can be diffused to all body tissuesParasitic – some have pharynx, others have no digestive tract at all because it has been digested by the host already

Movement Use cilia on epidermal cells to glide through water and sea floor Muscle cells are controlled by the nervous system –Allows for twist and turns so that they can react with the environment rapidly

Response Nervous system is more advanced than Cnidarian and Porifera Cephalization – simple brain Nerve cords that run throughout the body Ocelli – eye spots Cells that are sensitive to chemicals What about parasiticflatworms?

Planarian

Special Organs Simple kidney-like structure –Protonephridia controls water balance = osmoregulate Within the protonephridia are beating cilia, which draw water through – this looks like a flickering candle, so these cells are called flame cells

Planarian Organ System

Reproduction Asexual – –Binary fission, regenerationSexual –Free-living hermaphrodites – each individual can function as male and female Worms join in pairs and sperms are released into the other worm Eggs are laid in small cluster in open water –Parasitic Only reproduce sexually

Penis Fencing

3 Classes of Platyhelminthes TurbellariaTrematodaCestoda

Class Turbellaria Free-livingCarnivorous Regenerative powers Ex. planarians (freshwater)

Class Trematoda Known as “ Flukes ” Parasitic Found mostly in tropical areas People infected with blood flukes (Schistosoma) get extremely sick – often die. Intermediate host

(Schistosoma) Blood Flukes (Schistosoma)

IntermediateHost eggs grow into larvae eggs grow into larvae

Class Cestoda Ex. Tapeworms Long, flat, parasitic Head is called scolex and has several suckers and a ring of hooks Live in intestine and absorb food through body wall Behind scolex is neck region – constantly dividing – proglottids

Intermediatehost

Ecological Role Few positive effects on humans Food for bigger aquatic animals Help aerate soil

Extra: Binary Fission vs fragmentation (regeneration)

Budding Lateral outgrowth called a bud

Fragmentation