Final answer:
The major feature that Nematoda shares with Platyhelminthes is the presence of a pseudocoelom.
Step-by-step explanation:
Nematoda (roundworms) shares the major feature of pseudocoelom with Platyhelminthes. Both of these phyla have a body cavity that is partially lined by both mesoderm and endoderm tissue. The pseudocoelom provides structural support and allows for movement in these organisms.
In contrast, Platyhelminthes lacks a true coelom and instead has a solid body with no body cavity. They are acoelomates - organisms that lack a body cavity altogether.
Therefore, the major feature that Nematoda shares with Platyhelminthes is the presence of a pseudocoelom.