- (zoology) Any animal that has bilateral symmetry, collectively grouped in the clade Bilateria.
- 2005, N. S. Sharma, Continuity And Evolution Of Animals, page 91:
- Whatever the phylogenetic relationship among bilaterians and the diploblastic phyla, there was clearly a significant expansion in genomic information within the internode that connects the bilaterian clade to other branches.
- 2007, J. W. Valentine, Seeing ghosts: Neoproterozoic bilaterian body plans, Patricia Vickers-Rich, Patricia Komarower (editors), The Rise and Fall of the Ediacaran Biota, [page 372],
- Lacking body cavities and both circulatory and excretory organs, with subepidermal nerve plexuses and 'brains' represented, if at all, by thickened anterior commissures, and with simple reproductive set-ups, acoels come close to representing a model stem ancestor for bilaterians.
- 2010, José Xavier-Neto, Marcos Sawada Simoes-Costa, Rodrigo Abe Castro, Hozana Andrade Castillo, Allysson Coelho Sampaio, Ana Paula Azambuja, Chapter 1.1: Evolutionary Origins of Hearts, Nadia Rosenthal, Richard P. Harvey (editors), Heart Development and Regeneration, Volume 1, page 12,
- In most bilaterians, the gut musculature and most vascular muscles derive from the mesoderm.