Phylum Hemichordata Definition

It is contested if they are Phylum hemichordata, which indicates they are “half” or “part” chordates (Greek: hemi, half; chorde, cord). Typically, they are vermiform, solitary or colony enterocoelous coelomate creatures lacking normal nephridia and with an intra-epidermal nervous system.

Phylum Hemichordata Characteristics

• They are entirely marine, tubicolous, and solitary or colony.
• Their bodies are triploblastic, bilaterally symmetrical, vermiform, delicate, and fragile.
• The proboscis, collar, and trunk make up the body.
• Mucous glands on a body wall with a single layer of epidermis. lacks dermis.
• Protocoel, mesocoel, and metacoel are divisions of coelom enterocoelous.
• Previously thought to be the notochord, the buccal diverticulum is located in the proboscis.
• A full, U-shaped, or straight tube makes up the alimentary canal.
• One to several pairs of dorso-lateral pharyngeal gill slits are present when present. feeders with iliary filters.
• A dorsal heart and two longitudinal vessels—one dorsal and one ventral—combine to provide a straightforward, open, and well-developed circulatory system.
• Excretion through the proboscis’s lone glomerulus.
• Subepidermal nerve plexus make up the majority of this primitive form of neural system. hollow nerve cord in the dorsal collar.
• Most reproduction is sexual. Usually, the sexes are distinct. one to numerous pairs of gonads.
• Seawater external fertilisation
• A tornaria larva that is free to swim might undergo direct or indirect development.
• Balanoglossus, Saccoglossus, Rhabdopleura, Atubaria, and Dendrograptus are a few examples.

Phylum Hemichordata Classification

Hemichordata has been classified into 4 groups and has roughly 80 recognised species.

Class 1: Enteropneusta (Greek: enteron, gastrointestinal; pneustos, breathing)

• referred to as “acorn” or “tongue worm” often.
• Creatures that are solitary, free-swimming, or burrowers.
• A vermiform body that is lengthy and without a stalk.
• The proboscis, collar, and trunk make up the body; the collar lacks tentaculate arms (lophophore).
• Cylindrical and tapering proboscis.
• Straight gastrointestinal tract with mouth and anus at either ends. feed filters.
• Several pairs of gill holes in the form of a U.
• There are two pairs of hepatic caecas in the centre of the trunk.
• Sexes are distinct. Numerous, scan-like gonads
• With or without tornaria larva, development
• There is no asexual reproduction.
• Examples include Balanoglossus, Saccoglossus, Protoglossus, and Ptychodera.

Class 2. Pterobranchia (Gr., pteron, feather +branchion, gill)

• Solitary or colonial, sessile, tubicolous, and sedentary.
• Residing within secretory chitinous tubes.
• Short, compact body with an attaching stalk.
• Proboscis has ciliated tentacles to create water currents for ciliary feeding.
• Ciliated arms on the collar (lophophore).
• Never U-shaped; always one set of gill slits or none.
• With an u-shaped alimentary canal. The Dorsal Anus was situated close to the mouth.
• Distinct or combined sexes. gonads, either one or two.
• Direct development may or may not involve a larval stage that is free to swim.
• Asexual reproduction by some kind of budding

Order 1. Rhabdopleurida

• A stolon connects colonial zooids.
• Two tentaculated arms on the collar.
• No gill openings.
• Only one gonad.
• Example: Rhabdopleura, a single genus.

Order 2, Cephalodiscida

• A single or a group of zooids coexisting in an unrelated gelatinous casing.
• Collar featuring several tentacled arms.
• Present are only one set of gill slits.
• There is only one pair of gonads.
• Cephalodiscus and Atubaria are two examples.

Class 3. Planctosphaeroidea

• A few tiny, spherical, translucent, and pelagic larvae that are thought to be specialised tornaria of an unidentified hemichordate called Planctosphaera pelagica serve as the class’s representatives.
• Largely branching ciliary rings cover the larval body.
• The digestive tract is formed like an L.

Class 4. Graptolite

• Extinct colonial hemichordates are mostly recognised by the remains of their tubes.
• Both the Ordovician and Silurian eras include plenty of them.
• Every animal is kept in its own zooid.
• Their colony lifestyle and tubular chitinous structure indicate a kinship with Rhabdopleura.
• For instance, Dendrograptus.

References

• Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
• Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.