Most animals are Bilateria. ‘The bilaterians: building a body’ explains that Bilaterians have ‘bilateral symmetry’, meaning they have a single line of mirror-image symmetry running down the centre of the body. Bilaterians also have well-defined blocks of muscle for active movement, almost all have centralized nerve cords with an anterior brain, plus specialized sense organs concentrated at the front end. The evolution of bilaterian animals marked the rise of animals with active, powerful, and directed locomotion, able to burrow, crawl, or swim while facing their environment head-on with batteries of sense organs, leaving their waste products behind them. The importance of homeotic or Hox genes in body development is also described.