How do nematode and annelid body plans differ?

The term bilateria is used for describing a clade of the eukaryotic Kingdom Animalia in which the developing embryo has bilateral symmetry. This clade includes Acoelamate and three other lineages, namely, Deuterostomia, Ecdysozoa, and Lophotrochozoan.

Bilateria is that clade of animal phylogeny which comprises the soft-bodied invertebrates along with vertebrates.

The animals belonging to this lineage have bilateral symmetry along with the triploblastic body. The mesoderm layer in these animals is placed between the other two embryonic layers (endoderm and ectoderm).

Six distinct groups of invertebrate animals are present under Lophotrochozoan, annelids being one of them.

In biological science, body plan refers to the structural organization shared by a group of animals belonging to the same animal phylum. It would share some common features with closely related members, and some features would be particular to the phylum.

Annelids are animals with a coelomate type of body plan; that is, the body of these animals have several true cavities and proper body segmentation. In contrast to this body plan, the nematodes (members of the ecdysozoans group) have an unsegmented cylindrical body that lacks true coelom.