Vertebrates are defined by having backbones, a muscular system with bilaterally paired masses, and a central nervous system partly enclosed within the backbone. It has a distinct head, with a differentiated tubular brain and three pairs of sense organs (nasal, optic, and otic). The body is divided into trunk and tail regions. The presence of pharyngeal slits with gills indicates a relatively high metabolic rate. Although the vertebral column is perhaps the most obvious feature of present-day vertebrates, it was not present in the first ones, which probably had only a notochord (a longitudinal flexible rod of cells forms the supporting axis of the body).
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