Phylum Platyhelminthes

Phylum Platyhelminthes (Greek platy = flat, helminth = worm) includes animals such as Planaria, Taenia (tapeworm) and Fasciola (liver fluke). The body is flattened from top to bottom (dorso-ventrally), giving the group its common name, flatworms.

These animals are acoelomate and triploblastic, and have reached the organ-system level of organisation. Most members are endoparasitic (living inside a host), though some, like Planaria, are free-living. Parasitic forms bear hooks and suckers for attaching to the host's body, and their body surface is covered by a protective cuticle, while free-living forms are covered by cilia instead.

A digestive system is generally absent altogether in parasitic forms (since they absorb pre-digested nutrients from the host), while free-living forms have an incomplete digestive system following the blind sac plan. …