Hemichordate

What is Hemichordata?

Hemichordata is a phylum of ocean deuterostome organisms that is sometimes referred to as the echinoderms' sister group. Enteropneusta (acorn worms) and Pterobranchia are the two primary classes that occur in the Lower or Middle Cambrian. Planctosphaeroidea, a third class, is only recognized from the larva of one species, Planctosphaera pelagica. Pterobranchs are closely connected to the extinct Graptolithina class.

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Acorn worms are worm-like invertebrates that live alone. They are deposit feeders who reside in burrows, however other species are pharyngeal filter feeders, and the Torquaratoridae family are free-living detritivores. Numerous are very well recognized for producing halogenated pyrroles and phenols and accumulating them. Pterobranchs seem to be filter-feeders that live in a collagenous tubular form called a coenecium. They are mainly colonial.

Classification of Hemichordata

Kingdom: Animalia

Subkingdom: Eumetazoa

Superphylum: Deuterostomia

Phylum: Hemichordata

Phylum Hemichordata Examples

Hemichordata Examples: Balanoglossus, Atubaria, Rhabdopleura, Saccoglossus, Dendrograptus.

Balanoglossus

Scientific Name – Balanoglossus

Common Name – Acorn worm or Tongue worm

Class – Enteropneusta

Delle Chiaje discovered and named Balanoglossus clavigerus in 1829.

Balanos and glossus are two Greek words that were used to create their generic name. The term balanos alludes to the proboscis that protrudes from the collar and resembles an acorn-nut, hence the colloquial name "acorn worm." The term glossus relates to the shape of its collar, proboscis, and genital wings, which resembles that of an ox tongue, thus the common name "tongue worm."


Balanoglossus is a marine tubicolous or burrowing hemichordate that lives in the intertidal zone's shallow coastal waters. Soft, extended, cylindrical, worm-like, bilaterally symmetrical, and unsegmented body. It is separated into three sections: the protostome at the front, the middle collar, and the posterior long trunk with the terminal anus. Proboscis is a tongue-like outgrowth that forms on the skin. The collar is the muscular area below the proboscis stalk that holds the mouth ventrally.

The Trunk is Divided Into Three Sections:

  • The gonads and gill holes are located in the anterior branchiogenital area. The gonads are contained in narrow, flat lateral areas that create genital wings.

  • Externally, the middle hepatic region is defined by irregular elevations caused by sacculations generated by the intestine's extending hepatic caeca.

  • The posterior abdominal region progressively narrows and culminates in a terminal anus.

The coelom is an enterocoelous structure that is large and surrounded by coelomic epithelium.


The animal dwells in a U-shaped tube or burrows with two apertures and two vertical limbs. Even though the worm is lethargic and unaffected by external stimuli, cilia covering its body surface allow it to move around in its burrow. The proboscis has been the most active component of the body, and it assists in digging. By ingesting sand or mud, Balanoglossus feed on diatoms, protozoans, other microbes, and organic waste.


The lateral cilia lining the gill slits create a circulation of water that enters from the mouth, travels through the buccal cavity, gill slits, pharynx, and branchial sacs, and exits through the gill pores. This is termed as respiratory-cum-food current.


Balanoglossus' respiratory system consists of two parts: (i) branchial segment of the throat with gill-slits, and (ii) branchial sacs with gill pores.


Balanoglossus' blood circulatory system is either open or lacunar. It is made up of (i) colourless blood, (ii) a central sinus and a heart vesicle, (iii) spreading vessels (arteries and sinuses), and (iv) collecting vessels (veins).


The glomerulus or proboscis gland, which is located in front of the central sinus and projects into the proboscis coelom, is the excretory organ. Brown or yellow particles are excreted. The nervous system is primitive, similar to that found in coelenterates and echinoderms. Males and females dwell in separate tubes because the sexes are dioecious. Gametes are shed in seawater and combine there, resulting in external fertilisation. The tornaria larva is a free-swimming planktonic ciliated larva that develops in an indirect manner.

This does not reproduce asexually, but it does have a high regeneration capacity.

Saccoglossus

Scientific Name – Saccoglossus

Common Name – Tongue worm

Class – Enteropneusta

It is a classic enteropneust genus, with habitat, habits, and anatomy quite identical to Balanoglossus. It's a tubicolous tongue worm with a soft body that lives in spirally coiled burrows in the sea. The body is divided into three sections: proboscis, collar, and trunk. Hepatic caeca and genital wings are missing. Mature gonads are yellow in males and grey in females, with dorsolateral genital folds in the centre of the trunk marking their position externally.

Without a free-swimming tornaria larva, development is rapid. Saccoglossus pygmaeus is the world's smallest Enteropneusta species, measuring 2 to 3 centimetres in length.

Anatomy

A muscular arrangement distinguishes the body design of hemichordates. The anterior prosome, intermediate mesosome, and posterior metasome are the three sections of the anteroposterior axis.


Acorn worms have a worm-like body with three parts: an anterior proboscis, a middle collar, and a posterior trunk. The proboscis is a muscular, ciliated organ that helps animals move around and gather and transfer food particles. Between the proboscis and the collar is the mouth. The animal's trunk is the longest section of its body. The pharynx, which has gill slits (or pharyngeal slits), the oesophagus, a lengthy intestine, and a terminal anus are all contained within it. The gonads are also found there. Juvenile members of the Harrimaniidae family of acorn worms have a post-anal tail.


Pterobranchs' prosomes are developed into a muscular, ciliated cephalic shield that aids in the movement and secretion of the coenecium. The mesosome is divided into one pair of tentaculated arms (in the species Rhabdopleura) or numerous pairs (in the genus Cephalodiscus) for filter feeding. Asexual budding produces a metasome, or trunk, which includes a looped digestive system, gonads, and a contractile stalk that binds people to other individuals of the colony. Asexually formed individuals in the genus Cephalodiscus remain linked to the parent individual's contractile stalk till they have completed their growth. The zooids of the genus Rhabdopleura are constantly linked to the remainder of the colony through a stolon system.


They do have stomochord, a foregut diverticulum that was once assumed to be connected to the chordate notochord, although this is more likely the consequence of convergent evolution than homology. Some animals have a hollow neural tube (certainly in early life), which is likely a primordial characteristic which they share with Chordata and the rest of the deuterostomes.

Calcium carbonate biomineralization in several animals.

Development

Hemichordates, along with Echinoderms, make up the Ambulacraria, which are chordates' closest living evolutionary relatives among invertebrates. As a result, these sea worms are of tremendous importance in the study of chordate development's origins. Hemichordates come in a variety of shapes and sizes, with a modest range of embryological development. Hemichordates have traditionally been thought to evolve in two ways: directly and indirectly. Hemichordates are a phylum that includes two classes: enteropneusta and pterobranchs, both of which are types of marine worms.


Direct and indirect development are the two developmental mechanisms used by enteropneusta. An extended pelagic plankotrophic tornaria larva stage is included in the indirect developmental strategy, which indicates that such a hemichordate persists as a larval stage that relies on plankton until maturing into an adult worm. Rhabdopleura, a Pterobranch genus found in England, Plymouth, and Bermuda, has been examined extensively. 


The development of two well-studied hemichordata species, Saccoglossus kowalevskii and Ptychodera flava are described here. Ptychodera flava is a direct developer while Saccoglossus kowalevskii is an indirect developer. The majority of the information about hemichordate development has come from hemichordates that have developed directly.


Ptychodera Flava: S. kowalevskii has a similar early cleavage pattern to P. flava. The animal and vegetal poles of the embryo are included in both the first and second cleavages from the P. flava single-cell zygote. The embryo contains four blastomeres at the vegetal and animal poles after the third cleavage, which is equal and equatorial. The fourth division occurs mostly in the animal pole's blastomeres, which split both transversally and evenly to produce eight blastomeres. The four vegetal blastomeres split evenly but unequally, resulting in four large macromeres and four smaller micromeres. The embryo has reached the 16-cell stage once the fourth division has happened. Four vegetal micromeres, eight animal mesomeres, and four bigger macromeres make up the 16-cell embryo of P. flava. P. flava continues to divide until the blastula stage is completed and gastrulation begins. 


P. flava's animal mesomeres give birth to the larva's ectoderm, and animal blastomeres appear to contribute to these structures as well, though the exact contribution varies from embryo to embryo. The vegetal micromeres produce the internal endomesodermal tissues, while the macromeres produce the posterior larval ectoderm. Studies of the embryo's potential at various stages have revealed that P. flava blastomeres can give rise to tornaria larva at both the two and four-cell stages of development, implying that the destiny of these embryonic cells is unknown until after this stage.


Saccoglossus kowalevskii: S. kowalevskii eggs are oval in shape at first, but following fertilisation, they turn spherical. The initial cleavage occurs between the animal and vegetable poles and is usually equal, however, it might be unequal in some cases. The second cleavage, which leads to the embryo's four-cell stage, happens in a fairly equal manner from the animal to the vegetal pole, while it is possible to have an unequal division, as with the first. The cleavage of the eight-cell stage is latitudinal, meaning that each cell from the four-cell stage divides into two cells. The fourth division occurs first in the cells of the animal pole, resulting in eight non-radially symmetric blastomeres (mesomeres), and then in the four blastomeres of the vegetal pole, resulting in a level of four enormous blastomeres (macromeres) and four extremely small blastomeres (micromeres). 


To produce a 32 cell blastomere, the fifth cleavage happens first in animal cells and then in vegetal cells. In a similar order, the sixth cleavage completes a 64-cell stage, while the seventh cleavage completes the cleavage stage with a blastula with 128 blastomeres. This structure then undergoes gastrulation motions, which shape the body plan of the emerging gill slit larva, which in turn gives rise to the marine acorn worm.


Genetic control of dorsal-ventral hemichordate patterning: Many of the genetic markers identified in this group are also found in chordates or are homologous to chordates in some way, as much of the genetic work done on hemichordates has been done to compare with chordates. Studies of this sort have focused on S. kowalevskii, which, like chordates, has dorsalizing BMP-like factors such as BMP 2/4, which is similar to Drosophila's decapentaplegic DPP. Bmp2/4 expression starts on the ectodermal side of the embryo at the start of gastrulation and narrows down to the dorsal midline as gastrulation develops, but it is not expressed in the post-anal tail.


In the endoderm of gastrulating S. kowalevskii, the BMP antagonist chordin is also expressed. In addition to these well-known dorsalizing factors, S. kowalevskii contains other molecules known to be involved in dorsal-ventral patternings, such as a netrin that belongs to the netrin gene classes 1 and 2. Netrin, like the chemical SHH, is critical in chordate nervous system patterning, but S. kowalevskii only has one hh gene, and it appears to be expressed in a different place than where it is generally expressed in growing chordates along the ventral midline.

Classification

The Enteropneusta, often known as acorn worms, and the Pterobranchia, which includes graptolites, are the two classes of Hemichordata. Planctosphaeroidea is a suggested third class based on a single species known exclusively from larvae. There are roughly 120 extant species in this phylum. Hemichordata looks to be sister to the Echinodermata as Ambulacraria, with Xenoturbellida possibly being the most basal of the Echinodermata. Enteropneusta is paraphyletic since Pterobranchia can be generated from within it. Etacystis, an extinct organism, may have been a member of the Hemichordata, either within or closely related to the Pterobranchia. Hemichordata has 130 known species, and many more are being discovered, particularly in the deep water.

Structure

Hemichordata circulatory system- The development of two well-studied hemichordata animals species, Saccoglossus kowalevskii and Ptychodera flava, is described here. Ptychodera flava is a direct developer while Saccoglossus kowalevskii is an indirect developer. The majority of the information about hemichordata animals development has come from hemichordates which have developed directly. Some animals have a hollow neural tube (at least in early life), which is most likely a basic characteristic shared by Chordata and the rest of the deuterostomes.

Hemichordata Characteristics

Below Given are the General Characters of Hemichordata:

  • Hemichordata is a triploblastic, bilaterally symmetrical mammal.

  • They are only found in the sea.

  • They can live alone or in groups.

  • Hemichordata have a genuine coelom or body cavity.

  • The digestive tract is completed by an anus, which could be in the shape of a U or a straight tube.

  • In the proboscis, there is a buccal diverticulum.

  • Proboscis, Collar, and Trunk are the three regions of the body.

  • Open-type circulatory system.

  • Gills are used for respiration.

  • The excretory organ of the proboscis is the glomerulus.

  • There is a rudimentary nervous system.

  • Separation of sexes or unification of sexes.

Graptolites

Graptolites are a type of Palaeozoic fossil. They are colony animals that lived mostly from the Upper Cambrian to the Lower Carboniferous periods. Chaunograptus, a probable early graptolite, has been discovered in the Middle Cambrian.


The word graptolite originates from the Greek words graptos, which means "written," and lithos, which means "rock." Numerous graptolite fossils seem like hieroglyphics carved into the rock. Linnaeus considered them to be "images resembling fossils instead of actual fossils" at first. Recent research suggests that they live near or within the pterobranchs.

Graptolite Morphology

A rhabdosome is a graptolite colony with a random number of branches (stipes) originating from a single person. The zooid after that is enclosed in a tubular or cup-like structure (called a theca). There seem to be two sizes of theca in certain colonies, and it's been hypothesised that this is due to sexual dimorphism. A rhabdosome is a graptolite colony with a random number of branches (stipes) originating from a single person. The zooid after that is enclosed in a tubular or cup-like structure (called a theca). There seem to be two sizes of theca in certain colonies, and it's been hypothesised that this is due to sexual dimorphism. In order to identify graptolite fossils, the number of branches and the placement of the thecae is significant characteristics. A hacksaw blade's general form has been compared to theirs.


Dendroid graptolites are the most common tree-like formations (order Dendroidea). They were benthic animals with a root-like base that were anchored to the seafloor early in the fossil record.

Acorn Worms

Acorn worms, also known as Enteropneusta, are a Hemichordate invertebrate that is closely related to chordates. There are roughly 70 species of acorn worms in the globe, with Saccoglossus kowalevskii being the most studied. All of the species are benthic, meaning they eat on the ground or in the air. Some of these worms can grow to be rather large; one species has been known to reach 2.5 metres (nearly eight feet) in length, while most acorn worms were so much smaller. Balanoglossus, for example, is sometimes referred to as the tongue worm.

Pterobranchs

Pterobranchia is a worm-like group of tiny creatures. They're hemichordate, and they reside in hidden tubes on the ocean floor. Pterobranchia eats by using cilia connected to tentacles to filter plankton from the water. The group consists of approximately 30 extant species.


E. Ray Lankester erected the class Pterobranchia in 1877. It included only the genus Rhabdopleura at the time. Cephalodiscus, the second genus nowadays included in the order, had affinities in the direction of the Enteropneusta, according to the Challenger Report (Cephalodiscus) published in 1887. Pterobranchs appear to relate to the very same group as the extinct graptolites, according to electron microscope studies.

FAQs on Hemichordate

Q1. Why are Hemichordates Known as Half Chordates?

Ans. The name Hemichordate, which means "half chordate," comes from the fact that they share certain Chordate traits but not all. They are deuterostomes, with the first opening (the blastopore) becoming the anus throughout embryonic development, as opposed to protostomes, where it has become the mouth.

Q2. What Proof/Evidence Is Responsible for Linking Hemichordates and Chordates?

Ans. The conserved region in the genomes of these two worms offers the first evolutionary connection between hemichordates and chordates, and as a result, all deuterostomes — proving that humans and their less complex ancestors share a common ancestor.

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