Phylum Hemichordata- Characteristics, Classification, Examples

Phylum Hemichordata Definition

It is contested if they are Phylum hemichordate, which indicates they are “half” or “part” chordates (Greek: hemi, half; chorde: cord). Typically, they are vermiform, solitary or colony enterocoeles coelomate creatures lacking normal nephridia and with an intra-epidermal nervous system.

Phylum Hemichordata Characteristics

  • They are entirely marine, tubicolous, and solitary or colony.
  • Their bodies are triploblastic, bilaterally symmetrical, vermiform, delicate, and fragile.
  • The proboscis, collar, and trunk make up the body.
  • Mucous glands on a body wall with a single layer of epidermis, lacking dermis.
  • Protocoel, mesocoel, and metacoel are divisions of coelom enterocoelous.
  • A full, U-shaped, or straight tube makes up the alimentary canal.
  • One to several pairs of dorsolateral pharyngeal gill slits are present.
  • A dorsal heart and two longitudinal vessels—one dorsal and one ventral—combine to provide a straightforward, open, and well-developed circulatory system.
  • Excretion through the proboscis’s lone glomerulus.
  • Sub-epidermal nerve plexus make up the majority of this primitive form of neural system; hollow nerve cord in the dorsal collar.
  • Most reproduction is sexual. Usually, the sexes are distinct; one to numerous pairs of gonads.
  • Typically have external fertilization.
  • A tornaria larva that is free to swim might undergo direct or indirect development.
  • Balanoglossus, Saccoglossus, Rhabdopleura, Atubaria, and Dendrograptus are a few examples.

Phylum Hemichordata Classification

Hemichordata has been classified into 4 groups and has roughly 80 recognized species.

Class 1: Enteropneusta (Greek: enteron, gastrointestinal; pneustos, breathing)

  • Referred to as “acorn” or “tongue worm” often.
  • Creatures that are solitary, free-swimming, or burrowers.
  • A vermiform body that is lengthy and without a stalk.
  • The proboscis, collar, and trunk make up the body; the collar lacks tentaculate arms (lophophore).
  • Cylindrical and tapering proboscis.
  • Straight gastrointestinal tract with mouth and anus at either ends. feed filters.
  • Several pairs of gill holes in the form of a U.
  • There are two pairs of hepatic caecas in the center of the trunk.
  • Sexes are distinct. Numerous, scan-like gonads.
  • With or without tornaria larva, development.
  • There is no asexual reproduction.
  • Examples include Balanoglossus, Saccoglossus, Protoglossus, and Ptychodera.

Class 2. Pterobranchia (Gr., pteron, feather +branchion, gill)

  • Solitary or colonial, sessile, tubicolous, and sedentary.
  • Residing within secretory chitinous tubes.
  • Short, compact body with an attaching stalk.
  • Proboscis has ciliated tentacles to create water currents for ciliary feeding.
  • Ciliated arms on the collar (lophophore).
  • Never U-shaped; always one set of gill slits or none.
  • With an u-shaped alimentary canal. The Dorsal Anus was situated close to the mouth.
  • Distinct or combined sexes. gonads, either one or two.
  • Direct development may or may not involve a larval stage that is free to swim.
  • Asexual reproduction by some kind of budding

Order 1. Rhabdopleurida

  • A stolon connects colonial zooids.
  • Two tentaculate arms on the collar.
  • No gill openings.
  • Only one gonad.
  • Example: Rhabdopleura, a single genus.

Order 2. Cephalodiscida

  • A single or a group of zooids coexisting in an unrelated gelatinous casing.
  • Collar featuring several tentacled arms.
  • Present are only one set of gill slits.
  • There is only one pair of gonads.
  • Cephalodiscus and Atubaria are two prominent examples.

Class 3. Planctosphaeroidea

  • A few tiny, spherical, translucent, and pelagic larvae that are thought to be specialized tornaria of an unidentified hemichordate called Planctosphaera pelagica serve as the class’s representatives.
  • Largely branching ciliary rings cover the larval body.
  • The digestive tract is formed like an L.

Class 4. Graptolite

  • Extinct colonial hemichordates are mostly recognized by the remains of their tubes.
  • Both the Ordovician and Silurian eras include plenty of them.
  • Every animal is kept in its own zooid.
  • Their colony lifestyle and tubular chitinous structure indicate a kinship with Rhabdopleura.
  • For instance, Dendrograptus.


  • Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
  • Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.
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