Phylum Platyhelmenthes

Phylum Platyhelmenthes: Planarians, Flukes, & Tapeworms

• Simple acoeleomates
• Bilaterally symmetrical
• Most flattened "Dorsoventrally"

• Divided into 4 classes:

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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.

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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.

Flukes

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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.