Phylum Hemichordata: Definition, Characteristics, Classification, Examples

Phylum Hemichordata is generally considered the sister group of the phylum Echinodermata. The animals are marine deuterostomes. They are more or less worm-like, having the ancestral tripartite body of proboscis, collar, and trunk.

The name Hemichordata is derived from the words Hemi and Chorde (Gr., hemi, half; chorde, cord or string). Thus, hemichordata means they are ‘half‘ chordates.

There are about 90 known species in the phylum Hemichordata (Barnes et al., 1993).

Definition of Hemichordata

Phylum Hemichordata is a group of marine deuterostomes having proboscis, collar, and trunk in the body with pharyngeal gill slits and intra-epidermal nervous system.

Diagram of Phylum Hemichordata
Figure: Phylum Hemichordata

Characteristics of Hemichordata

  • The animals are exclusively marine.
  • They remain either solitary or form colonies.
  • The body is soft, fragile (devoid of the skeleton), worm-like, triploblastic, and bilaterally symmetrical. It is divided into three parts: the proboscis, collar, and trunk.
  • Epidermis is ciliated.
  • Coelom is enterocoelous (i.e., arising from the wall of the embryonic gut) and divided into protocoel, mesocoel, and metacoel.
  • The alimentary canal is straight or U-shaped.
  • The buccal or pre-oral diverticulum, called the stomochord (earlier considered the notochord), is present in the proboscis.
  • The pharyngeal gill slits are usually present and are in one to several pairs.
  • Nervous system is of a diffuse and primitive type and, in some cases, has a hollow dorsal nerve cord in the collar.
  • A simple and open circulatory system is present.
  • The excretory organ consists of a single glomerulus.
  • Reproduction is mostly sexual. Sexes are separate (gonochoristic) or united.
  • Fertilisation occurs externally in the seawater.
  • Presence of a free-swimming tornaria larva in the life cycle.

Examples of Hemichordata

Balanoglossus, Saccoglossus, Ptychodera, Atubaria, and Rhabdopleura.

Classification of Hemichordata

Phylum Hemichordata has been divided into four classes.

  • Class 1. Enteropneusta (e.g., Balanoglossus)
  • Class 2. Pterobranchia
    • Order 1. Cephalodiscida (e.g., Atubaria)
    • Order 2. Rhabdopleurida (e.g., Rhabdopleura)
  • Class 3. Planctosphaeroidea (e.g., Planctosphaera pelagica)
  • Class 4. Graptolite (e.g., Dendrograptus)

Class 1. Enteropneusta (Gr., enteron = gut; pneustos = breathed)

  • A group of marine animals commonly known as acorn or tongue worms.
  • These are solitary animals, either free-swimming or burrowing in marine habitats.
  • Elongated, worm-like body.
  • The body is composed of the proboscis, collar, and trunk.
  • The collar is short and lacks tentaculated arms (lophophore).
  • Proboscis is more or less conical.
  • Straight alimentary canal with mouth and anus at opposite ends.
  • Several pairs of U-shaped gill-slits. These gill-slits help in filter feeding and respiration.
  • Two pairs of hepatic caeca are found in the middle of the trunk.
  • The glomerulus is well developed.
  • Reproduction occurs through both asexual and sexual methods.
  • Sexes separate. Gonads are numerous and sac-like.
  • Development with or without tornaria larva.
  • Examples: Balanoglossus, Saccoglossus, and Pyrchodera.

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

  • These are sedentary, tubicolous (live inside secreted chitinous tubes) animals.
  • They can exit in solitary or colonial forms.
  • Proboscis have ciliated tentacles, which produce ciliary feeding currents of water.
  • Collar bears two or more tentaculated arms (lophophore).
  • Trunk short and sac-like.
  • One pair of gill-slits or none. Gill-slits are never U-shaped.
  • Alimentary canal U-shaped, with the dorsal anus lying near the mouth.
  • Glomerulus is poorly developed.
  • Sexes are separate or united. A single or one pair of gonads are present.
  • Development is usually direct. Some species exhibit a free-swimming larval stage in their life cycle.

Order 1. Cephalodiscida

  • Solitary or several zooids living in a common gelatinous house. These zooids are unconnected.
  • Collar with many tentaculated arms.
  • One pair of gill-slits is present.
  • Gonads one pair.
  • Examples: Atubaria, Cephalodiscus.

Order 2. Rhabdopleurida

  • Colonial, zooids are connected by a stolon.
  • Collar has two tentaculated arms.
  • Gill-slits absent.
  • Gonad only one.
  • Example: Rhabdopleura.

Class 3. Planctosphaeroidea

  • Represents only some small, rounded, transparent, and pelagic larvae.
  • These larvae have branching arborescent ciliated bands on their surface.
  • U-shaped alimentary canal.
  • Example: Planctosphaera pelagica.

Class 4. Graptolite

  • These are extinct colonial hemichordates.
  • Abundant in the Ordovician and Silurian periods.
  • Each animal remains in a zooid.
  • Example: Dendrograptus.

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