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.

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.
Namita Das
Namita Das

Namita Das is a development professional with considerable research experience in public health nutrition and teaching experience in biology and nutrition. To discuss her current profession, she has been working as a consultant in nutrition and health at the Better World Foundation in Jharkhand, India. She has research experience on tribal traditional food, nutrition, malnutrition, and health from the Cultural Research Institute, Backward Classes Welfare, and Tribal Development Department, Government of West Bengal. She has completed her graduation in Nutrition Honours with zoology and physiology from Belda College and post-graduation in Nutrition and Dietetics with a specialization in Medical Nutrition Therapy from Vidyasagar University, West Bengal, India.

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