Amphioxus

Amphioxus also called lancelet,  any of certain members of the invertebrate subphylum Cephalochordata of the phylum Chordata. Amphioxi are small marine animals found widely in the coastal waters of the warmer parts of the world and less commonly in temperate waters. Both morphological and molecular evidence show them to be close relatives of the vertebrates. The theory is that the Amphioxus split from vertebrates about 520 million years ago, their genomes hold clues to their evolution and have been heavily researched.