The Phylum Platyhelminthes contains over 20,000 free-living and parasitic species of acoelomate animals called flatworms. In flatworms, the body that is flattened dorsoventrally, with the mouth and genital pore usually located in a ventral position. The space between the gut and outside is filled with mesodermal muscle fibers and undifferentiated parenchyma. Although fluid-filled spaces in the parenchyma serve as a hydrostatic skeleton for support and to aid in internal transport, the animals lack a body cavity, which is why they are called acoelomate. Most free-living flatworms have a gastrovascular-type digestive system (a mouth is present but no anus), while parasitic forms generally have no digestive system.

Flatworms have a centralized nervous system consisting of pair of cerebral ganglia and longitudinal nerve cords connected to transverse nerves. The excretory system (absent in some forms) consists of two lateral canals with protonephridia bearing flame cells. Although many flatworms are free-living, the phylum includes some very important parasitic species as well.

In terms of reproduction, flatworms can reproduce sexually or asexually. Most species are monoecious but practice cross fertilization. Many freshwater turbellarians can reproduce asexually by fission in which the animal simply divides into two halves, each of which regenerates the other half. In some turbellarians (as it is in most other animals), the yolk that provides nutrition for the developing embryo is containing within the egg cell itself, a condition described as endolecithal. In the monogeneans, trematodes and cestodes (as well as in a few turbellarians), yolk is contributed by cells released from organs called yolk glands, and the eggs are therefore described as ectolecithal. Development may be direct or indirect.