Subphylum Cephalochordata (Lancelets)

Subphylum Cephalochordata (Lancelets)

Cephalochordates — represented by the lancelet, Amphioxus (Branchiostoma) — are small, elongated, fish-shaped marine animals that, unlike the tunicates, retain all four chordate hallmark features throughout life, not just as a transient larval stage: a notochord running the full length of the body (persisting into the adult, rather than being replaced by a vertebral column, since cephalochordates are invertebrate chordates with no true backbone), a dorsal tubular nerve cord, numerous pharyngeal gill slits, and a post-anal tail.

Like Urochordata, Cephalochordata possesses an endostyle in the floor of its pharynx, performing the same ciliated, mucus-secreting, food-trapping role in filter feeding, and again regarded as homologous to the vertebrate thyroid gland. …