Phylum Hemichordata: Classification, Characteristics & Examples

Phylum Hemichordata, widely known as a sub-phylum, is generally considered the sister group of the echinoderms. This phylum is small and contains a few hundred species, and it is one of the most important groups of species. 

• Phylum Hemichordata consists of animals with distinct structures and forms. 
• This species of phylum includes two main classes, namely Pterobranchia and Enteropneusta.
• It also includes a third class of species, namely Planctosphaeroidea.
• These animals possess definite shapes and sizes with organs internally linked.

Phylum Hemichordata includes sense organs, nervous system, and excretory organs. These species live in burrows and are deposit feeders.

• Echinodermata, Chordata and hemichordate are all considered ancestor groups.
• The common name of the Phylum Hemichordata is acorn worm.
• The habitat of this species is the entire ocean.
• These animals produce both sexually and asexually.

Key Terms: Phylum Hemichordata, Hemichordata, Echinodermata, Chordata, Species, Animals, Nervous Systems, Excretory Organ, Budding, Marine animals, Phylum chordata, Acorn worms

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What is Hemichordata?

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The term Hemichordata was derived from the Greek word "hemi", meaning "half", and chorde", meaning "string", thus, "half-chordate". It shows that this phylum has some characteristics of the Chordata imbibed in it while others do not.

• Hemichordata is a phylum that contains a small group of marine animals which are worm-like.
• These animals are equipped with organ system level of organisation.
• This category of animals is also called Acorn Worms and was previously grouped under the phylum Chordata. 
• Bateson considered that this group is a subphylum of the phylum Chordata.
• Balanoglossus is the scientific name of this phylum.

Later, a separate phylum, Hemichordata, was formed since those organisms did not have a post-anal tail or the notochord, which is one of the significant features of the animals belonging to phylum Chordata.

• Some renowned scientists, such as Van der Horst, Dawydoff, Marcus, and Hyman, give it an independent status.
• The body plan of the phylum consists of the anterior prosome, the intermediate mesosome, and the posterior metasome.
• Animals found in this species is triploblastic, enterocoelomate, and bilaterally symmetrical.

The tabulated details about Phylum Hemichordata are as follows:

Phylum Hemichordata

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Classification of Phylum Hemichordata

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The phylum hemichordate includes almost a few hundred species and is divided into four classes, namely the following-

Enteropneusta

The characteristics of Enteropneusta are as follows:

• Enteropneusta are usually solitary, free-swimming animals.
• They have an elongated, vermiform body with no stalk.
• These animals have a straight alimentary canal.
• They lack asexual reproduction and contain numerous pairs of U-shaped gill slits.

Pterobranchia

The characteristics of Pterobranchia are as follows:

• Pterobranchia are sedentary, may be solitary or colonial, and tubicolous animals.
• Their bodies are compact, with stalks for attachment, and live in secreted chitinous tubes.
• They contain a U-shaped alimentary canal and contain either one or no gill slits.
• Anus dorsal lies near the mouth.
• They undergo asexual reproduction by budding.
• These are further divided into two orders - Rhabdopleura and Cephalodiscida.

Graptolite

The characteristics of Graptolite are as follows:

• Graptolites have a tubular chitinous skeleton.
• They live in colonies.
• These are extinct hemichordates.
• They show a close similarity with Rhabdopleura.

Planctosphaeroidea

The characteristics of Planctosphaeroidea are as follows:

• The planctosphaeroidea alimentary canal is L-shaped.
• Their larval body is covered by ciliary bands.
• They contain some specialized tornaria larvae.

Classification chart of Phylum Hemichordata is as follows:

Kingdom Animalia

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Subkingdom Eumetazoa

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Superphylum Deuterostomia

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Phylum Hemichordata

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Class Enteropneusta

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Class Graptolithina

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Class Pterobranchia

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Class Planctosphaeroidea

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Characteristics of Hemichordata

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Important characteristics of Hemichordata are as follows:

• The organisms in the phylum Hemichordata are bilaterally symmetric and triploblastic.
• The animals can live alone or in colonies and are mostly tubicolous.
• They are exclusively marine animals that perform respiration with the help of gills.
• Their circulatory system is open-type, quite simple, and well-developed.
• Hemichordata uses a glomerulus situated in the proboscis gland as the excretory organ. 
• Their bodies are cylindrical, soft, fragile, vermiform, and divided into anterior proboscis, collar, and trunk.
• These animals have a single-layered epidermis, which is their body wall.

These animals under the phylum Hemichordata have a primitive type of nervous system, which mainly consists of a subepidermal nerve plexus. Moreover, they have a hollow dorsal nerve cord.

• Reproduction is mainly sexual, and the sexes are separate; gonads are one to several pairs.
• Development is mostly indirect through free swimming tornaria larva.
• Direct development is also seen in some cases.
• They are coelomate animals whose fertilization is external.

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Balanoglossus

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Balanoglossus is an acorn or tongue worm that is considered significant in the field of zoology as it is an “evolutionary link” between invertebrates and vertebrates. 

• It is a deuterostome that looks like Ascidians or sea squirts.
• Balanoglossus consists of branchial openings or gill slits.
• These species do have a stomochord, which means their gut chord is enclosed by the collar.
• The superior part of the body consists of a notochord.

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Things to Remember 

• Phylum Hemichordata is a species that contains an organ-system level of organisation, also known as the sister group of Echinodermata.
• These animals can be solitary or live in colonies, exclusively marine animals.
• They perform respiration with the help of gills, whose bodies are divided into anterior proboscis, collar, and trunk.
• Hemichordata are considered Ambulacraria, which are chordates that live close to other invertebrates.
• There are 130 known species, and many more are being discovered in the deep water.

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