Animals possessing coelom are called coelomates, e.g., annelids, molluscs, arthropods, echinoderms, hemichordates and chordates. In some animals, the body cavity is not lined by mesoderm, instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called pseudocoelom and the animals possessing them are called pseudocoelomates, e.g.,
aschelminthes. The animals in which the body cavity is absent are called acoelomates, e.g., Platyhelminthes.

They have dorso-ventrally flattened body, hence are called flatworms. Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with organ level of organisation.
Examples: Taenia (Tapeworm), Fasciola (Liver fluke).