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

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

Planarian and assorted marine flatworms…

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

Life Cycle of the Liver Fluke

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

Class Cestoda (Tapeworms) Parasitic flatworms Can grow to enormous lengths (up to 20m) Parasitize mostly vertebrate hosts Attach head named scolex to intestinal walls of host, and abosrb host's nutrients Following 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.

Tapeworms cont’d…