The Collaborative International Dictionary of English v.0.48:

Amphioxus \Am`phi*ox"us\, n. [NL., fr. Gr. 'amfi` + ? sharp.]
   (Zool.)
   A fishlike creature (Amphioxus lanceolatus), two or three
   inches long, found in temperature seas; -- also called the
   lancelet. Its body is pointed at both ends. It is the
   lowest and most generalized of the vertebrates, having
   neither brain, skull, vertebr[ae], nor red blood. It forms
   the type of the group Acrania, Leptocardia, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

amphioxus
    n 1: small translucent lancet-shaped burrowing marine animal;
         primitive forerunner of the vertebrates [syn: lancelet,
         amphioxus]