Animal Kingdom

Phylum Hemichordata

This is a phylum that contains marine deuterostome animals. They are sometimes considered as the sister group of phylum Echinodermata. Phylum Hemichordata is a small phylum with only a 100 known species, with animals having a worm-like appearance. Some species may be solitary or some occur in colonies. Let us study them further.

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Phylum Hemichordata

Balanoglossus-gigas

(Source: Britannica)

These animals are also called as Acorn Worms. They were initially grouped under Phylum Chordata. But, research has proven that none of these organisms does not have a post-anal tail or even the notochord, which is a distinguishing feature of chordates. Hence, these animals have been given a separate phylum, called the Hemichordata.

They are typically found in oceans, living on the seafloor. They vary greatly in size, ranging from a few millimetres to almost one and a half meters.  Balanoglossus gigas, a hemichordate species can be of 1.5 meters in length. These species feed on small organic particles. They can be either filter feeders or substrate eaters.

Most of the organisms belonging to this phylum have a modified proboscis, due to which they have got the name Acorn worms. These animals show a few chordate characteristics but are not completely chordates. Hence they have been given the name Hemichordata (Half chordates). Another notable feature of these organisms is the threefold division of the body, with the preoral lobe, collar, and a trunk. The collar bears the tentacles in a few species. The trunk has the digestive and reproductive organs.

Learn more about Phylum Arthropoda here.

Characteristic Features of Phylum Hemichordata

  • Hemichordata is bilaterally symmetrical and triploblastic animal.
  • They are exclusively marine animals.
  • They can be solitary or in colonies.
  • Hemichordata have a true body cavity or coelom.
  • The digestive tract is complete with an anus and can be in the form of a U shaped tube or straight.
  • A buccal diverticulum is present in the proboscis.
  • Body is divided into three regions – Proboscis, Collar and Trunk.
  • Circulatory system in open type.
  • Respiration occurs through gills.
  • The proboscis has a glomerulus, which is the excretory organ.
  • A primitive nervous system is present.
  • Sexes are separate or united.
  • Reproduction is mostly sexual reproduction.
  • Fertilization is external.
  • Development is mostly indirect. But a few species exhibit direct development.

Learn more about Phylum Annelida here.

Classification of Phylum Hemichordata

Phylum Hemichordata is again divided into two classes:

  • Enteropneusta: This class includes acorn worms. They have a vermiform body and are found on sandy beaches near seas in warm climates.
  • Pterobranchia: They have a tube-dwelling and live in deep sea waters. They are bottom dwellers who attache to other organisms for their survival.

    Examples

Acorn-worm-Hemichordate1

(Source: Wikipedia)

  • Balanoglossus
  • Saccoglossus

Learn more about Phylum Porifera here.

Solved Questions For you

Q: Balanoglossus belongs to which phylum?

Ans: It belongs to Phylum Hemichordata.

Q: In hemichordates, the body is divided into three regions. Name them.

Ans: The three regions of the body are Proboscis, Collar and Trunk.

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