|n. pl.||1.||(Zool.) The lowest class of Vertebrata, including only the Amphioxus. The heart is represented only by a simple pulsating vessel. The blood is colorless; the brain, renal organs, and limbs are wanting, and the backbone is represented only by a simple, unsegmented notochord. See Amphioxus.|
Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.