What features distinguish Platyhelminthes from Annelida?

Platyhelminthes, Nematodes and Annelids differ from each other on the basis of presence and absence of body cavity called coelom. The coelom is a fluid-filled cavity that lie between the digestive tube and the outer body tube which is surrounded by mesodermal tissues.

In Platyhelminthes, the body does not have any cavity and hence, they are flat. Due to the absence of coelom, these invertebrates are also called Acoelomates.

Nematodes, on the other hand, do have a space in the body between the digestive tract and the body wall. But it is not lined with tissue and hence, is not considered to be a true coelom. So, nematodes are sometimes referred to as Pseudocoelomates.

Unlike Platyhelminthes and Nematodes, Annelids have a true coelom, i.e. they possess a body cavity between the digestive tube and the external body wall which is lined with tissue.