The bilateria or bilaterians are animals with bilateral symmetry, i.e., they have a front ("anterior") and a back ("posterior") as well as an upside ("dorsal") and downside ("ventral"); therefore they also have a left side and a right side. In contrast, radially symmetrical animals like jellyfish have a topside and downside, but no identifiable front or back.

The bilateria or bilaterians are animals with bilateral symmetry, i.e., they have a front ("anterior") and a back ("posterior") as well as an upside ("dorsal") and downside ("ventral"); therefore they also have a left side and a right side. In contrast, radially symmetrical animals like jellyfish have a topside and downside, but no identifiable front or back.