The Platyhelminthes Skeletal System:
Platyhelminthes are characterized as being either soft-bodied or lacking cuticles and don't have the rigid skeletal support system found in a more complex phylum. Most Platyhelminthes come in flat ribbion or leaf like shapes. They are usually unsegmented and invertebrate animals.
Trematode (Blood Fluke)
The Blood Fluke is part of the invertebrate class of trematodes (sub-class of Playtyhelminthes), and its symmetrical body is covered with a non-cellular cuticle. This cuticle gives the Blood Fluke a flattened leaf or ribbon-like shape. This cuticle also acts as a tough outer barrier protecting the Blood Fluke from digestive enzymes.
Planarian
The Planarian's skeletal support comes in the form of connective tissue known as the parenchyma, which is located between the body wall and the gut. Despite this connective tissue, like all Platyhelminthes, the Planarian lacks an actual skeleton. Instead the Planarian makes use of its cilia for locomotive purposes and has two layers of muscles to help it move around.
Turbellaria
The Turbellaria, like the Planarian, has a space between its skin and its gut that is filled with connective tissue which is reinforced by collagen fibers that act as a skeletal support system for the Turbellaria. Some Turbellaria have also been known to have spicular skeletons on top of the collagen fibers which give them the appearance of annulations.