(Science: zoology) The lowest class of vertebrata, including only the Amphioxus. The heart is represented only by a simple pulsating vessel. The blood is colourless; the brain, renal organs, and limbs are wanting, and the backbone is represented only by a simple, unsegmented notochord. See Amphioxus.
alternative forms: Leptocardii.
Origin: NL, fr. Gr. Small – the heart.