Round worms, which have body cavities that are partially lined with mesoderm, are classified as:

Explanation

A pseudocoelomate is an organism with a body cavity called a pseudocoelem that is not derived from the mesoderm, as in a true coelom, or body cavity. In invertebrates, the pseudocoelom is filled with fluid that lies between the mesoderm layer of the external body wall and the endoderm layer of the gut. But this is not a true coelem as it originates from the blastocoel, not the archenteron of Gastrula, which is the true site of origin of coelem. Roundworms in the phylum Aschelminthes are pseudocoelemates.