Phylum Hemichordata Definition
It is contested if they are Phylum hemichordate, which indicates they are “half” or “part” chordates (Greek: hemi, half; chorde: cord). Typically, they are vermiform, solitary or colony enterocoeles 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, lacking dermis.
- Protocoel, mesocoel, and metacoel are divisions of coelom enterocoelous.
- A full, U-shaped, or straight tube makes up the alimentary canal.
- One to several pairs of dorsolateral pharyngeal gill slits are present.
- 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.
- Sub-epidermal 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.
- Typically have external fertilization.
- 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 recognized 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 center 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 tentaculate 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 prominent examples.
Class 3. Planctosphaeroidea
- A few tiny, spherical, translucent, and pelagic larvae that are thought to be specialized 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 recognized 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.
- 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.