Practice question

Phylum Chordata is diagnosed by four synapomorphies expressed at some life stage: dorsal supporting notochord of vacuolated cells with sheath, dorsal hollow nerve cord derived by neurulation of ectoderm, pharyngeal slits or pouches for filter feeding and respiration, and post-anal tail with segmented muscles beyond anus for propulsion. Ventral solid nerve cord with segmental ganglia characterizes protostomes like annelids and arthropods, reflecting different neurogenesis and axon growth patterns. Hollow dorsal cord permits expansion into brain and spinal cord, distinguishing chordates neurologically and enabling cephalization central to vertebrate complexity.