Phylum Platyhelmenthes Planarians, Flukes, & Tapeworms Simple acoeleomatesSimple acoeleomates Bilaterally symmetricalBilaterally symmetrical Most flattened "Dorsoventrally"Most flattened "Dorsoventrally" Divided into 4 classes:Divided into 4 classes:

Class Turbellaria Most free-living and non parasiticMost free-living and non parasitic Include planariansInclude planarians Lack circulatory and gas-exchange organs.Lack circulatory and gas-exchange organs. Simple excretory structures called flame cells that maintain osmotic balance.Simple excretory structures called flame cells that maintain osmotic balance. Is cephalized and has eye-spots which are sensitive to lightIs cephalized and has eye-spots which are sensitive to light Feeds through ventral feeding tube called a pharanxFeeds through ventral feeding tube called a pharanx Can reproduce sexually, or asexually through regenerationCan reproduce sexually, or asexually through regeneration

Planarian and assorted marine flatworms…

Class Trematoda (Flukes) Similar in form to turbillariansSimilar in form to turbillarians Many are parasitic. Example of parasitic fluke is the asian liver flukeMany are parasitic. Example of parasitic fluke is the asian liver fluke Many have complex life cycles including multiple hosts and alternation of generations.Many have complex life cycles including multiple hosts and alternation of generations.

Life Cycle of the Liver Fluke

Liver Flukes Cont’d… Liver Fluke encysted in a liver Liver Fluke

Class Cestoda (Tapeworms) Parasitic flatwormsParasitic flatworms Can grow to enormous lengths (up to 20m)Can grow to enormous lengths (up to 20m) Parasitize mostly vertebrate hostsParasitize mostly vertebrate hosts Attach head named scolex to intestinal walls of host, and abosrb host's nutrientsAttach head named scolex to intestinal walls of host, and abosrb host's nutrients Following scolex is a series of repeating structural units called proglottidsFollowing scolex is a series of repeating structural units called proglottids Mature proglottids containing thousands of eggs are released with the host's feces to infect other organisms.Mature proglottids containing thousands of eggs are released with the host's feces to infect other organisms.

Tapeworms cont’d…