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Phylum Hemichordata Definition, Classification, Characteristics, Examples

Hemichordata (Gr., hemi, half; chorde, cord) means they are 'half' of 'part' chordates, a fact that is disputed. They are generally vermiform...

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This article writter by MN Editors on November 20, 2021

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Phylum Hemichordata Definition, Classification, Characteristics, Examples
Phylum Hemichordata Definition, Classification, Characteristics, Examples

Definition of Phylum Hemichordata

Hemichordata (Gr., hemi, half; chorde, cord) means they are ‘half’ of ‘part’ chordates, a fact that is disputed. They are generally vermiform single, or colonial enterocoelous coelomate species with an intra-epidermal nerve system as well as a pre-oral, with or without gill-slits, and without the typical Nephridia.

Characteristics of Phylum Hemichordata

  • They are only marine, colonial or solitary, usually tubicolous.
  • They have a soft body, delicate as well as vermiform and unsegmented. bilaterally symmetrical, and triploblastic.
  • The body can be divided into collar, proboscis and trunk.
  • The body wall is a single-layered epidermis as well as mucous glands. No dermis.
  • Coelom enterocoelous, divided into mesocoel, protocoel as well as metacoel.
  • Buccal diverticulum was previously thought as the notochord, is present within the proboscis.
  • The canal for the alimentary tract is straight or complete tube.
  •  Dorso-lateral pharyngeal gill-slits, when present, one to several pairs. Ciliary filter feeders.  
  •  Simple, transparent as well-developed, circulatory system that includes two dorsal hearts and two long vessels that run longitudinally, one ventral and the other ventral.
  • Excretion through one glomerulus located within the proboscis.
  • Primitive type nerve system comprised of primarily Subepidermal Nerve plexus. Dorsal collar nerve cord hollow.
  • The majority of reproduction is sexual. The sexes are usually separated. They usually form a couple.
  • External fertilization in seawater.
  • Development can be direct or indirect using an open-swimming tornaria larva.
  • Examples: Balanoglossus, Saccoglossus, Rhabdopleura, Atubaria, Dendrograptus.

Classification of Phylum Hemichordata 

Hemichordata contains around 80 known species, and is divided into four classes.

Class 1- Enteropneusta (Gr., enteron, gut +pneustos, breathed)

  • Often referred to as “acorn” or “tongue worm”.
  • Individual, free-swimming or animals that burrow.
  • Vermiform, elongated body without a stalk.
  • The body is comprised of the proboscis and collar and trunk; the collar is without any tentaculated arms(lophophore).
  • The Proboscis is cylindrical, and the tapering.
  • Straight alimentary canal, mouth and anus at opposing ends. Filter food intake.
  • Many pairs of gill-slits with U-shaped.
  • Two pairs of hepatic caeca found on the inside of the tree.
  • Separate sexes. The scans are numerous and scan-like.
  • Development without or with tornaria larva.
  • In the absence of sexual reproduction, there is no.
  • Examples: Balanoglossus, Saccoglossus, Protoglossus, Ptychodera.

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

  • Sedentary, solitary , or colonial tubicolous, sessile and colonial.
  • Inside secreted chitinous tubes.
  • Body compact, short, with a stalk to attach.
  • Proboscis that has tentacles ciliated to produce ciliary feeding streams of water.
  • Collar that has ciliated arms(lophophore).
  • A pair of gill-slits, or none at all, never U-shaped.
  • The alimentary canal is shaped like a U. Anus dorsal located close to the mouth.
  • Separate or unisexual males or females. You can choose to have a single or a couple.
  • Development direct, could or might not contain an open-swimming larval stage.
  • Budding reproduction is asexual and sexually transmitted one form or another.

Order 1. Rhabdopleurida

  • Colonial, zooids linked with an Stolon.
  • Collar with 2 arms.
  • There are no Gill-slits.
  • Single gonad.
  • Example: single genus Rhabdopleura.

Order 2. Cephalodiscida

  • The Solitary or several zooids living unconnected within a common gelatinous case.  
  • Collar with several arms.
  • A single pair of gill-slits are present.
  • One pair of gonads is present.
  • Examples: Cephalodiscus, Atubaria.

Class 3. Planctosphaeroidea

  • This class can be seen in only a few tiny, round transparent, pelagic larvae believed to be tornaria specialized of an unknown hemichordate called Planctosphaera Pelagica.
  • Larval body with a brittle cover. the ciliary bands.
  • The alimentary canal has a L-shape.

Class 4. Graptolite

  • Extinct colonial hemichordates, mostly identified by fossilized structures in their tube.
  • They are plentiful in these rocks in the Ordovician and Silurian times.
  • Each animal is housed within the Zoooid.
  • Their tubular chitinous skeleton as well as colonial habits suggest an affinity to Rhabdopleura.
  • Example: Dendrograptus.

Distribution and Habitat of Phylum Hemichordata

As with other Chordates, Hemichordates are marine organisms. They are distributed widely and are located in oceans all over the world. Although they constitute all marine invertebrates they are located in various habitats within the marine environment.

For instance, members of this class Enteropneusta (also called Acorn Worms) are typically found in the seashore as well as shallow marine areas, where they reside in burrows that resemble U-shaped (E.g. Balanoglossus the clavigerus or Saccoglossus ruber make use of their proboscis muscles to make burrows in mud or sand). But, some species are found in the shells of stones, or in depths as high as 10,000 feet in which they can swim in a wide range of conditions.

The members belonging to the group Pterobranchia On contrary, are typically found in the deepest regions of ocean waters , where they reside in hidden tubular structures referred to as coenecia. Most species are located within the Southern Hemisphere and live as suspension feeders.

Members belonging to the genera Rhabdopleura as well as Cephalodiscus are two species that can form coenecia. They are nonchitinous tubes which are usually produced by the cephalic tuberculum. These tubes are different in their shape and size and are linked with an tolon. Each coenecial space can be home to an entire colony of 15 zooids that are joined.

Anatomy of Phylum Hemichordata

Hemichordates in the phylum Hemichordata are soft-bodied, worm-like invertebrates. The bodies (which are elongated and cylindrical) are separated into three primary parts that comprise the collar, trunk and the proboscis. The proboscis can be found in the top on the body. In enteropneusts, the proboscis has a short length with an conical projection.

In Pterobranchs However, the tube is extended with a shield-like look. In Pterobranchs the tube-like structure has been sucked out of portions that are part of the proboscis. The area between the proboscis and trunk is called the collar. The acorn species (Enteropneusts) the collar is simply a rings-like portion that extends posteriorly of the proboscis.

For Pterobranchs The collar is made up of ciliated tentacles that resemble arms (2-9 arms). The trunk is the posterior portion of the organisms. It’s U-shaped within Pterobranchs and is home to the vast majority of the internal organs.

Proboscis

In all phylum Hemichordata the anterior portion of the body (proboscis) is home to the heart vesicle, as well as the central sinus, and the buccal diverticulum. For Pterobranchs as previously mentioned the proboscis expands into a shield-like shape.

Alongside releasing the tubular coenecium inside which they live The proboscis is also able to play vital roles in the motion of the organism within the tube. In addition to housing vital organs, the proboscis also involved in the burrowing process and in locomotion.

By peristaltic motions the proboscis is extended into the air, which allows it to act as an anchor that draws the remainder parts of your body. This action is crucial in burrowing. The skin of the acorn is covered in cilia which aids in locomotion.

Collar Region

The mouth portion, a an element of the coelom and the stomochord may be located in the collar region in the body. In contrast to acorn species the collar region in Pterobranchs is also composed of tentacles that are ciliated. These are crucial structures involved in feeding. By using tentacles, these species are able to create tiny water currents , and utilize their cilia to trap food particles that are trapped in the current.

* The stomochord which is similar to the notochord, originates from the the buccal cavity. It’s a hollow and stiff structure believed to support the heart-glomerulus structure.

* In the proboscis Hemichordates are able to access the gland (proboscis gland) that plays a role in mucus production.

In relation to the manner in which the species feed on, those of the phylum Hemichordata can be classified as sediment feeders or suspended feeders. Acorns located in marine habitats that are shallow are sediment feeders. They take nutrients from organic material that is found in the environment around them. They are sediment feeders and consume organic matter that is contained within the soil.

Certain species, especially those that live deep water depths, can be classified as suspension feeders because they have access to organic material while swimming in a free manner.

A majority Pterobranchs however, on the contrary side, are suspended-feeders which draw organic matter through producing water currents. According to some research the proboscis covered in mucus could be a key element during feeding by catching food particles.

To digest food enzymes that are that break down food are released from the sacs of the liver. After eating, the worm expands its posterior edge out from its Burrow (U-shaped Burrow) to expel waste material (castings).

* Before maturing into adults, the larval stages of these worms consume plankton and other organic matter for a period of weeks or even months.

* Other organs and structures involved in feeding the feeders for filtering, such as the gill slits, the pre-oral organ of ciliary (POCO) and the lining of the pharynx, which is cylindrical in shape. These cilia form the lateral gill bars and the pre-oral organ pump water across the dorsal portion of the proboscis and into an opening in the mouth.

When water is pumped through the mouth, and out through the slits, particles get trapped and are able to settle within the primary and second gill bar.

Trunk Region

The majority of the internal organs found in these organisms are found within the area of the trunk. This is the extended part of Hemichordates. The trunk comprises three distinct regions, which comprise the Branchiogenital region and the middle caecal area, and the post-abdominal region.

Also called the thoracic region The branchiogenital area of the trunk contains the gonads. The like sac-like organs (gonads) create bulges that are known as Genital ridges. They are located between the genital ridges on the top of the trunk, are two rows of gill openings.

If you observe closely one can identify the gill pore as well as the gill sacs. The hepatic part in the trunk shorter and is home to a set of pouches lateral to the caeca hepatic. These pouches play a crucial function in digestion. The remainder of the intestinal tract (a simple tubular intestinal tract) is located in the abdominal region posterior within the trunk.

Vascular and Nervous System

A steady circulation of blood is vital for proper functioning of different muscles that are involved in the proboscis’s movement, among numerous other tasks. For the circulation of blood, Hemichordates have in place an anterior blood vessel that moves blood to the posterior region as well as an anterior dorsal blood vessel through which blood flows from the anterior.

A simple or common heart that resembles an open vertebrate heart located in the proboscis , just above the stomochord’s end. It is composed of a muscular ventral wall as well as an artery of blood that runs within the heart sac as well as the the top of the diverticulum. Two main blood vessels (dorsal and ventral vessels) are located at the rear of the heart. This is then used to pump and force blood through the small vessels.

While gases exchanged in the pharyngeal openings, waste substance is removed through”glomeruli” (blood sinuses) located at the base of the proboscis. The waste materials that are filtered out into the coelom within the region eventually get eliminated through pores on the inside of the proboscis.

* The circulatory system that is found in the phylum Hemichordata is not able to efficiently transport oxygen. It nevertheless plays crucial roles in the elimination of waste materials. This is why it is also kidney.

Since they don’t have an internal nervous system The nerve tissue on the neck region is radiated into the neural network of the epidermis as well as the outer covering. In different species, the method by how this network is distributed is different.

In a few species, the nerve system is located in the lower part of the epidermis ciliated. It is distinguished by dorsal as well as ventral nerve tracts aswell the nerve plexus. In certain species the dorsal nerve is as a tubular structure and may also contain massive nerve fibers.

Reproduction and Life Cycle of Phylum Hemichordata

Pterobranchs

Although Pterobranchs aren’t as often seen as Enteropneusts (acorn kinds), studies have identified two ways of reproduction in these species.

This includes:

Asexual reproduction 

Pterobranchs are able to reproduce sexually by budding. This leads to the creation of offspring that are genetically identical, called Zooids. In Cephalodiscus the new zooids are introduced to the colony through budding out of an embryonic disc in the mother.

In the process, many Zooids are created inside the coenecium. For Rhabdopleura the buds form at the bottom of the zooids, giving the birth of new individuals. In time a septum begins growing and separates the tubes.

In some instances Zooids can leave the colony, and then create the new tube to another location to produce the formation of a new colony.

* The zooids that are produced by budding could be gonochoristic or hermaphroditic.

Sexual reproduction

Contrary to asexual reproduction reproduction in the phylum Hemichordata requires fertilization of eggs in order to create embryos. After fertilization, eggs that are rich in yolks are kept within the brood chamber for approximately 7 (7) weeks. In this time, the eggs undergo holoblastic circular, and equal the cleavage process to create homogeneously ciliated larva.

The larvae are usually extended with a tapering rear end. For Rhabdopleura, larvae also display an orange-yellow color, brown-colored pigmentation and a prominent ventral depression. In a time span of several minutes up to 24 hours, free larvae swim up until they meet solid objects or substrates.

When they come across an appropriate surface or substrate that they can crawl over, they become encased in a colorless oval cocoon. For up to a couple of weeks or months. If the conditions are right they can transform into an adult Zooid.

Within a few days following the metamorphosis occurs, the cocoon is broken when the zooid begins to form an echinecium, in which the new colony will be established.

Enteropneusts

Asexual reproduction

Unlike members of the class Pterobranchia, asexual reproduction in these species may occur through fragmentation/fission (architomy). In this kind of sexual reproduction, an individual is subjected to transverse fission, and split into two pieces (the anterior and posterior sections). The two parts will then regenerate the part that was missing and matures into an individual.

* Although the majority of Pterobranchs are sexually active, some species have been found as dioecious (separate male and female genders).

* Fertilization of eggs takes place externally.

Enteropneusta

Most Enteropneusts are dioecious which means they have distinct genders. Based on the species, females may lay a large number of eggs (e.g. female Saccoglossus Kowalskii may produce between 200 and 1000 Oocytes). Eggs are fertilized externally and form the zygotes which are then surrounded by a thick vitelline-like membrane.

In certain species (e.g. Saccoglossus kowalevskii) The development skips the larval stage and the eggs then hatch into worms that grow to mature organisms. This is called direct development. In others (e.g. Balanoglossus or Ptychodera species) The development process is indirect and involves an embryonic stage.

For the species mentioned above, studies have shown them to have non-yolky eggs. Following fertilization externally the zygote goes through several divisions before producing larvae that are that are covered in cilia. As the organism grows the cilia are tightly woven to form a single band wrapping around the mouth, and finally connecting at the apical plates. This is crucial in determining the direction of propulsion.

In the following phase in development, the ciliary bands is beginning to receding and the body grows longer due to the circular constriction of the ciliary bands develops into the form of a collar. This permits the three distinct parts in the body to be evident. The larva is then able to develop further, changing into a juvenile worm which grows into an adult Worm.

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