a subphylum of lower chordate animals, as opposed to the subphylum of Craniata, or Vertebrata. Acrania have the following characteristics: the head section of the body is not separate; there is no skull (hence the name); the whole body is segmented, including some internal organs, the excretory system, and the sexual glands; the sensory organs are primitive (there are only sensory cells, lying in the skin and along the neural tube); and there is no heart, only a pulsating ventral blood vessel. There are two families. The family Branchiostomatidae includes 19 species, of which the best-known is the lancelet. The family Amphioxididae comprises three species. These three species are small (up to 1 cm) and live at great depths in the equatorial zone of the oceans.