NOTE 1: These are two separate phyla, but usually considered together.

Note 2: The following is a cladistic classification

Hemichordata

• Enteropneusta: Acorn worms
• Pterobranchia: sacciform body, U-shaped gut. Aggregating or colonial.

Chordata

• Urochordata
  • Ascidiacea: Ascidians, sea squirts.
  • Thaliacea: Tunicates or salps
  • Appendicularia: Also called Larvacea, since adults retain many larval characteristics including notochord and muscular tail.
  • Sorberacea: Adults retain dorsal nerve tube.
• Cephalochordata: Lancets (amphioxus)
• Vertebrata
  • Anamniotes
    • Apoda
      • Agnatha: Jawless fishes
      • Conodontia: conodonts. Extinct
      • Myxini: hagfish
      • Pteraspidomorphi (Diplorhina): extinct bony fish
      • Cephalospidomorphi (Monorhina): lampreys
      • Gnathostomata: vertebrates with jaws
      • Chondrichthyes: Cartilagenous fishes: Sharks, skates, rays
      • Osteichthyes: Bony fishes
    • Tetrapoda
    • Amphibia: Frogs, toads, salamanders, etc.
  • Amniotes
    • Sauropsida
      • Anapsida
        • Testudinae
      • Diapsida
        • Archosauromorpha
          • Aves
          • Crocodila
          • Ornithischia
          • Saurischia
        • Ichthyopterygia
        • Sauropterygia
        • Lepidosaura
          • Squamata
    • Synapsida
      • Pelycosauria
      • Therapsida
      • Mammalia

Synopsis of Hemichordata:

About a hundred species, 75-80 of which are enteropneusts. Bilateral deuterostomes, with vermiform or saccate body. Fundamentally trimeric, with prosome, mesosome, metasome, each with coelomic compartments (prosocoel, mesocoel, metacoel). Solitary (Enteropneusts) or colonial (Pterobranchs). Feeding by ciliated pharyngeal slits. Open circulatory system, with "heart vescicle" in the proboscis. Hemichordates have a unique excretory organ called a glomerulus formed of fingerlike outpocketings of the peritoneum and blood sinuses in the proboscis. Adult nervous system is netlike plexus lying among the bases of the epithelial cells outside the basement membrane. Enteropneusts have a dorsal "neurochord", absent in pterobranchs. Reproduction is asexual in some enteropneusts and most pterobranchs. Sexual reproduction is gonochoristic, eggs and sperm shed by spawning, although some brooding is reported in pterobranchs.

Synopsis of Urochordata (tunicates):

3,000 species in four classes. All are suspension feeders. Bilaterally symmetrical, in larva if not in adult. All use mucus-covered pharyngeal gill slits for feeding. Water flows into mouth and pharynx via incurrent siphon, through gill slits and out into atrium, thence out through excurrent siphon. A ciliated groove called the endostyle lies on floor of pharynx (this groove accumulates iodine, hinting that it may be the phylogenetic precursor to the vertebrate thyroid). All secrete a tunic which may be leathery or gelatinous. Circulatory system is best developed in the ascidians. It consists of a ventral heart and an open circulatory system. Nervous system of adults is reduced to dorsal ganglion. Reproduction is both asexual and sexual. Of most taxonomic interest is the larval form which has a notochord, a dorsal hollow nerve tube, and a muscular tail, all typical chordate characteristics.

Synopsis of Cephalochordata:

Body plan shows interesting characteristics intermediate between vertbrates and invertebrates. A notochord is present, as is a dorsal hollow nerve tube. Body wall musculature is characteristically vertebrate, arranged in chevron-like myotomes. A caudal fin is present. Feeding is much like in ascidians, using mucus-covered gills and an atrium. An iodine-accumulating endostyle is also present. Gut has an antero- ventral digestive cecum (hepatic cecum) viewed as the precursor to the vertebrate liver (and pancreas?). Circulation is closed. No heart is present, but the overall pattern of the circulatory system is distinctly vertebrate. Excretion is via solenocytes (flame cells) like those of platyhelminthes, though this is thought to be convergeant evolution. Eye spots are randomly distributed in the dorsal nerve tube of Amphioxus. Reproduction is sexual, with gonochoristic individuals. Embryology is typically vertebrate, given an isolecithal egg, and includes formation of the nerve tube from a dorsal neural plate.