Phylum

This phylum is made up of marine deuterostome creatures. They are sometimes referred to as the Echinodermata’s sister group. Phylum Hemichordata is a small phylum with only 100 recognised species and animals that resemble worms. Some species are solitary, while others live in colonies. These animals have some chordate traits, but they are not fully chordates. As a result, they were given the name Hemichordata (Half chordates). The threefold division of the body, with the preoral lobe, collar, and trunk, is another distinguishing trait of these species. 

Characteristics of phylum Hemichordata

• They are entirely marine, solitary or colony, and tubicolous in nature.
• Soft, fragile, vermiform, unsegmented, bilaterally symmetrical, and triploblastic, their body is soft, fragile, vermiform, unsegmented, bilaterally symmetrical, and triploblastic.
• Proboscis, collar, and trunk are the three parts of the body.
• Mucous glands in a single-layered epidermis on the body wall. There is no dermis.
• Coelom enterocoelous, with protocoel, mesocoel, and metacoel divisions.
• Buccal diverticulum, formerly known as the notochord, is a structure found in the proboscis.
• The alimentary canal is a U-shaped tube that is straight and complete.
• When present, one to many pairs of dorso-lateral pharyngeal gill-slits. Filter feeders for the iliary system.
• A dorsal heart and two longitudinal veins, one dorsal and one ventral, comprise a simple, open, and well-developed circulatory system.
• A solitary glomerulus in the proboscis is responsible for excretion.
• The primitive nervous system is mostly made up of the subepidermal nerve plexus. The nerve cord of the dorsal collar is hollow.
• Examples are such as  Balanoglossus, Saccoglossus, Rhabdopleura, Atubaria, Dendrograptus.

Classification of Hemichordata :

Enteropneusta

• It’s also known as “acorn” or “tongue worm.”
• Animals that are solitary, free-swimming, or burrowing.
• The body is elongated and vermiform, with no stalk.
• The proboscis, collar, and trunk make up the body; the collar lacks tentaculated arms (lophophore).
• Proboscis is tapered and cylindrical.
• The alimentary canal is straight, with the mouth and anus at opposite ends. Feeding through a filter
• There are a lot of U-shaped gill slits.
• In the middle of the trunk, there are two pairs of hepatic caeca.
• The sexes are separated. Numerous, scan-like gonads.
• Tornaria larvae or no tornaria larvae
• There is no asexual reproduction.
• Balanoglossus, Saccoglossus, Protoglossus, and Ptychodera are some examples.

Pterobranchia

• Tubicolous, sedentary, solitary or colonial, sessile
• Living inside chitinous tubes that have been secreted.
• The body is small and short, with a stalk for attachment.
• Water ciliary feeding currents are produced by Proboscis with ciliated tentacles.
• Arms with ciliated edging on the collar (lophophore).
• It’s either one set of gill slits or none at all; they’re never U-shaped.
• The alimentary canal is in the shape of a U. Anus dorsal, close to the mouth.
• Separate sexes vs. unified sexes Gonads can be purchased individually or in pairs.
• A free-swimming larval stage may or may not be included in direct development.
• Budding is a sort of asexual reproduction.

Planctosphaeroidea

• A few small, spherical, translucent, and pelagic larvae, thought to be specialised tornaria of some unknown hemichordate called Planctosphaera pelagica, characterise the class.
• Ciliary bands with many branches cover the larval body.
• The alimentary canal is in the shape of an L.

Graptolite

• Extinct colonial hemichordates whose tubes have been preserved as fossils.
• During the Ordovician and Silurian periods, they are abundant.
• Each animal is kept in its own zooid.
• Their colony behaviour and tubular chitinous skeleton are similar to Rhabdopleura.
• Dendrograptus is one example.

Conclusion

This phylum is made up of marine deuterostome creatures. They are sometimes referred to as the Echinodermata’s sister group. They live on the seafloor and are usually found in oceans. They come in a wide range of sizes, from a few centimetres to about one and a half metres. However, research has shown that none of these organisms lack a post-anal tail or the notochord, which is a characteristic of chordates. As a result, these animals have been assigned their own phylum, the Hemichordata. Balanoglossus gigas is a hemichordate that can grow to be 1.5 metres long. Small organic particles are the main food source for these organisms.