Flatworms and Roundworms

Section 28.3

Flatworms

The largest group of acoelomate worms Contain a mesoderm Has tissues organized into organs Bilaterally symmetrical Flat, ribbon-like body Have a highly branched gastrovascular cavity Dissolved substances move through the body by

diffusion Most do not have a respiratory or circulatory system

Flatworms, continued…

Turbellaria Free-living Mostly marine Planarians – freshwater turbellarians

Cestoda Parasitic tapeworms Anterior end contains suckers and hooks Body increases in length by producing segments called

proglottids that break off during reproduction Trematoda

Flukes Endoparasites (inside) or ectoparasites (outside) May have complex life cycles with more than 1 host

Planaria

Marine Flatworm

Tapeworm

Fluke

Roundworms

Also called nematodes Pseudocoelomates – fluid movement

acts as a simple circulatory system Have a one-way digestive tract

Parasitic roundworms

Three sources of human infection:1. Ascaris – eggs in human waste end up in

soil, enter through ingestion, end up in respiratory system

2. Trichinella – passed to humans through undercooked pork, causes a disease called trichinosis

3. Necator (hookworms) – larvae can penetrate the soles of bare feet and enter the bloodstream

Ascaris

Trichinella

Hookworms