Tunicates and Lancelets

Tunicates and lancelets are the two invertebrate groups most closely related to vertebrates, both classified within or beside the phylum Chordata because they share a flexible rod shaped notochord at some stage of life. Tunicates, including sea squirts and free swimming pyrosomes, filter feed by pumping water through a sac like body and, as larvae, briefly show the tadpole like tail and nerve cord typical of chordates before most species settle and lose them as adults. Lancelets keep a simple fish shaped body and a notochord running the length of the body throughout their life, making them a living reference point for how the vertebrate body plan may have first evolved. Neither group has a backbone, but both illustrate the transition from simple filter feeding ancestors toward the vertebrate lineage.