Phylum Platyhelminthes

20,000 species. Bilateral symmetry. Flattened dorso-ventrally. Triploblastic. Cephalized !!!

Phylum Platyhelminthes Small flatworms with no permanent gut cavity. Free-living, marine and brackish water. Class Turbellaria Order Acoela

Phylum Platyhelminthes Class Turbellaria Order Acoela Order Tricladida Gut with three branches. Marine, freshwater, and terrestrial.

Phylum Platyhelminthes Class Turbellaria Order Acoela Order Tricladida Order Polycladida Gut with many branches. Large marine flatworms. Often brightly colored.

Turbellarians How do they feed themselves? Gut simple or complex. Partially digested food transferred throughout gut then taken up by gastrodermis.

Turbellarians How do they reproduce and develop? Asexual reproduction. Transverse fission. Able to regenerate lost body parts.

Turbellarians How do they reproduce and develop? Sexual reproduction Simultaneous hermaphrodites

Persian Carpet Flatworm …swimming

Pseudobiceros hancockanus

Netted Flatworm

Phylum Platyhelminthes – THE FOLLOWING ARE NOT MARINE Class Turbellaria Order Acoela Order Tricladida Order Polycladida Class Trematoda Digenetic flukes. Most are endoparasites. Suckers to attach to hosts.

Phylum Platyhelminthes Class Turbellaria Order Acoela Order Tricladida Order Polycladida Class Trematoda Class Cestoda Tapeworms. Endoparasites. No digestive tract.

Tapeworm Structure

Cestodes How do they reproduce and develop? Fig. 33 Usually involves more than one host. Variable life cycles. Proglottid drops off and is excreted by definitive host. zygotes develop in environment. Zygote is ingested by second host and it encysts in tissues which are consumed by the definitive host.