A subphylum or phylum of chordates that contains only the lancelets, a group of small invertebrate marine fishlike animals. There are 30 known extant species in two genera—Branchiostoma (Amphioxus) and Epigonichthys (Asymmetron). About 5–15 cm long, lancelets have gill slits and a notochord that persist into adulthood, the notochord extending the whole length of the body and providing the main skeletal support. The pharynx is modified for filter feeding (see endostyle). Molecular evidence has established the lancelets as the earliest diverging (i.e. most basal) of all chordate animal groups, rather than the tunicates (see urochordata).