Phylum Hemichordata forms a small phylum of the animal kingdom with only 130 known species, however, their importance cannot be neglected. Phylum Hemichordata involves marine deuterostome animals and is related to chordates. Phylum Hemichordata is very important for the study of vertebrates. They are considered as connecting links between vertebrates and invertebrates.

Hemichordata is also referred to as Acorn Worms. They are generally found in oceans. They typically live on the seafloor. In this article, we will provide detailed information about Phylum Hemichordata. Scroll down to learn more!

Definition of Phylum Hemichordata

Hemichordata is a lesser-known phylum of the animal kingdom. Hemichordates are usually vermiform, solitary, or colonial enterocoelous coelomate animals with an intra-epidermal nervous system and a pre-oral with or without gill-slits and without typical nephridia. In some DNA based studies of evolution, the hemichordates are considered very close to echinoderms than to true chordates. This can be considered by the fact that larvae of some hemichordates resemble very much like echinoderms.

Hemichordata is formed by two words “Hemi” means “half” and “chordata” means “notochord” This is also called half chordata.

Fig: Structure of Hemichordata

What is the History of Phylum Hemichordata?

Hemichordata is a small phylum of around \(130\) species. In \(1825\) Eschscholtz a German Zoologist discovered the first animal of hemichordata and named it Ptychodera and regarded it as a holothurian. And another Zoologist Delle Chiaje in \(1829\) added another member Balanoglossus clavigerus to the phylum.

The fossil record of graptolites, which are one group of hemichordates, is very well known and is often used to correlate rocks.

Characteristics Features of Phylum Hemichordata

The characteristics of Phylum Hemichordata are mentioned below:

1. Habit and Habitat: The creatures of this phylum are marine animals and they mostly live-in burrows in sand or mud. They are also live-in algae or beneath stones.
2. Symmetry and Body Organisation: They are deuterostomes bilaterally symmetrical and triploblastic. They possess an organ system level of organization.
3. Body Formation: They have soft, cylindrical, and unsegmented bodies. Their body is divided into three parts: proboscis, collar, and trunk. The body is a true coelom, but the true notochord is absent.
4. Digestive System: Animals of this phylum have a complete digestive system.
5. Respiration: Respiration in Hemichordata takes place through branchial openings or several pairs of gill slits or through the general body structure.
6. Circulatory System: They have an open circulatory system that contains a dorsal heart.
7. Excretory System: They have a proboscis gland.
8. Nervous System: They have a primitive nervous system consisting of an intraepidermal nerve plexus.
9. Sensation: Sensory cells of the epidermis act as sense organs and help in sensation.
10. Reproduction: The animals of this phylum are mainly sexual. They have separate sexes. They show indirect development with tornaria larvae.

Classes of Phylum Hemichordata

The phylum Hemichordata is divided into four classes. The different classes are:

1. Enteropneusta

1. They are also known as “acorns” or “tongue worms”.
2. They are free-swimming and burrowing animals.
3. The body is elongated and vermiform with no stalk.
4. The body is made of three divisions: the proboscis, collar, and trunk.
5. They have straight alimentary canal, mouth, and anus at opposite ends.
6. Respiration takes place through numerous pairs of gill-slits.
7. They have two pairs of hepatic caeca present in the middle of the trunk.
8. The reproduction takes place in separate sexes. They have numerous gonads.
9. The development takes place with or without tornaria larva.
10. They show mainly sexual reproduction.
11. Examples: Balanoglossus, Saccoglossus, Protoglossus, Ptychodera.

2. Pterobranchia

1. They form colonies and individuals are connected by stems or stolons.
2. They are less than a millimeter long.
3. They have short shield shaped bodies instead of elongated bodies.
4. They have branched tentacles to collect small food particles from water.
5. They create and live within a network of tubes known as coenecium.
6. They have collar bearing ciliated arms also known as lophophore.
7. They have one pair of gill-slits for respiration.
8. The alimentary canal is U-shaped and the anus dorsal is present near the mouth.
9. They have separate or united sexes and a single or one pair of gonads.
10. They show direct development. They may or may not include a-swimming larval stage.
11. They show asexual reproduction by budding.
12. They are divided in three genera Rhabdopleura, Cephalodiscus, and Atubaria.

3. Planctosphaeroidea

1. This class of hemichordata represents few small, rounded, transparent and pelagic larvae.
2. They are specialized tornaria of some unknown hemichordata termed as Planctosphaera pelagic.
3. The larval body is covered by extensively branched ciliary bands.
4. They have an L-shaped alimentary canal.

4. Graptolite

1. They are extinct colonial hemichordates which were found from the fossil structures of their tubes.
2. They were abundant in the Ordovician and Silurian periods.
3. Each animal of this class is housed in a zooid.
4. The tubular chitinous skeleton and colonial habits of graptolites show an affinity with Rhabdopleura.
5. Example: Dendrograptus.

Examples of Phylum Hemichordata

Below we have provided some of the examples of Phylum Hemichordata:

1. Balanoglossus

Fig: Balanoglossus

2. Saccoglossus

Fig: Saccoglossus

3. Protoglossus

Fig: Protoglossus

4. Ptychodera

Fig: Ptychodera

Interesting Facts

The longest species in phylum hemichordata is the Balanoglossus gigas of Brazil, reaches \({\rm{4}}{\rm{.9}}\,{\rm{ft }}\left( {{\rm{1}}{\rm{.5}}\,{\rm{m}}} \right)\) in length.

Summary

Phylum Hemichordata is a not so familiar phylum among people. It is a small phylum in the animal kingdom with about \(130\) species. Animals of this phylum are invertebrates. This phylum was earlier placed as a subphylum of Chordata but is now considered as a separate phylum under non-Chordata. They are mainly marine animals and live-in burrows. They are worm-like animals with soft bodies mostly elongated.

The body of a Hemichordata is divided into three parts: the proboscis, collar, and trunk. Some species of Hemichordata are very close to echinoderms. They are also considered as a link between invertebrates and vertebrates.

FAQs on Phylum Hemichordata

Q.1. What are two common characteristics between Chordates and Hemichordates?
Ans: The two common characteristics between chordates and hemichordates are the presence of tubular nerve cords and pharyngeal slits.

Q.2. What are the characteristics of Phylum Hemichordata?
Ans: a. The creatures of this phylum are marine animals and they mostly live-in burrows in sand or mud. They are also live-in algae or beneath stones.
b. They are deuterostomes bilaterally symmetrical and triploblastic. They possess an organ system level of organization.
c. Their body is divided into three parts: proboscis, collar, and trunk. The body is a true coelom, but the true notochord is absent.

Q.3. What are the classes of Phylum Hemichordata?
Ans: The classes of Phylum Hemichordata are: Enteropneusta, Pterobranchia, Planctosphaeroidea and Graptolite.

Q.4. Explain Phylum Hemichordata with an example.
Ans: Phylum Hemichordata is not a familiar phylum of the animal kingdom. The body of the animals of this phylum is made of three divisions: preoral robe, collar, and trunk. This phylum includes worms like animals. The creatures of this phylum are mainly marine. Example Balanoglossus, Saccoglossus, etc.

Q.5. Why is Hemichordata excluded from the chordates?
Ans: The hemichordates were excluded from the chordates because they do not have notochord or a post-anal tail in their body.

Q.6. What is the common name of an animal in Phylum Hemichordata?
Ans: Acorn worm is a common name of an animal in Phylum Hemichordata.

Q.7. State the importance of Phylum Hemichordata.
Ans: The importance of Phylum Hemichordata is listed below:
1. Phylum Hemichordata is very important for the study of vertebrates.
2. They are considered as connecting links between vertebrates and invertebrates.

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