Overview
Phylum Platyhelminthes - Flatworms
Flatworms are unsegmented and bilaterally symmetrical. There are about 25,000 known species of flatworms worldwide, many are free-living in marine, freshwater and terrestrial environments, while others are parasites. Free-living species are often brightly coloured. Parasitic species can be very damaging to their hosts, as shown by several species living in humans. A troublesome free-living terrestrial example is a New Zealand flatworm which was accidentally introduced to Ireland and Britain, where it is an important predator of earthworms.
There are four major groups of flatworms:
• Turbellaria (mostly free-living flatworms, some parasitic)
• Trematoda (endoparasitic flukes, as adults)
• Monogenea (ectoparasitic flukes)
• Cestoda (endoparasitic tapeworms)