Despite lacking a bony skeleton, vertebral column or spine, a skull, and a distinct brain, Acrania are close relatives of vertebrates (Vertebrata) and share several characteristics with them as a basal group of Chordata. Thus, Acrania have a notochord, a dorsal nerve cord, and a muscular postanal tail. Only a few species exist, and all of them are marine chordates of slender, lanceolate shape. The lancelets are the only living representatives of Acrania.