Phylum Hemichordata – Definition, Classification, Characteristics, Examples

• Phylum Hemichordata, also known as Acorn Worms, is a small but significant phylum within the animal kingdom. Originally considered a subphylum under Phylum Chordata, further research revealed distinct characteristics that set them apart from chordates, leading to the establishment of a separate phylum. The name “Hemichordata” is derived from the Greek words “hemi,” meaning “half,” and “chorde,” meaning “string,” emphasizing their partial resemblance to chordates.
• Hemichordata encompasses a group of marine animals with a worm-like appearance and a high level of organizational complexity in their organ systems. They are regarded as the sister group of Echinodermata, another important phylum. Previously classified under Phylum Chordata, Hemichordata was later reclassified due to the absence of key features such as a post-anal tail and a notochord, which are defining characteristics of chordates.
• Early classifications considered Hemichordata to be the lowest subgroup within Phylum Chordata, while recent scientists, including Van der Horst, Dawydoff, Marcus, and Hyman, recognized its distinct organization and granted it independent status as an invertebrate phylum. Nevertheless, the name Hemichordata has been retained for this group. J. Muller was the first to describe the Tornaria larva of Balanoglossus, contributing to our understanding of these organisms.
• With approximately 130 known species, Phylum Hemichordata may be small in terms of species diversity, but its significance cannot be underestimated. These marine deuterostome animals are closely related to chordates and play a crucial role in the study of vertebrates. They serve as vital connecting links between vertebrates and invertebrates, offering valuable insights into the evolutionary history and development of these animal groups.
• Acorn Worms, as they are commonly called, are primarily found in oceanic environments, inhabiting the seafloor. Their ecological niche and unique characteristics make them fascinating subjects of scientific inquiry. The study of Phylum Hemichordata provides detailed information about their anatomy, physiology, behavior, and evolutionary relationships, contributing to our broader understanding of the animal kingdom.
• In conclusion, Phylum Hemichordata stands as a distinct phylum within the non-chordate group, separate from Phylum Chordata. Acorn Worms, as members of Hemichordata, exhibit both similarities and differences to chordates, making them an important group for scientific research. By delving into the intricacies of this phylum, we gain valuable insights into the evolutionary connections between invertebrates and vertebrates, expanding our knowledge of the natural world.

Definition of Phylum Hemichordata

Phylum Hemichordata is a small marine phylum of worm-like animals known as Acorn Worms. They are distinct from chordates and serve as important connecting links between vertebrates and invertebrates.

Characteristics of Phylum Hemichordata

Phylum Hemichordata exhibits the following characteristics:

1. Habitat: Hemichordates are exclusively marine organisms, with some species being solitary while others form colonies. They are commonly found living in tubular structures or burrows in sand, mud, algae, or beneath stones.
2. Body Structure: Their bodies are soft, fragile, and vermiform (worm-like) in shape. They lack segmentation and are bilaterally symmetrical. The body can be divided into three distinct regions: the proboscis, collar, and trunk.
3. Body Wall: The body wall of Hemichordata consists of a single-layered epidermis, without a dermis. Mucous glands are present in the epidermis.
4. Coelom: They possess an enterocoelous coelom, which is divided into three parts: the protocoel, mesocoel, and metacoel.
5. Buccal Diverticulum: The proboscis of Hemichordata contains a buccal diverticulum, previously mistaken for a notochord. This structure aids in feeding.
6. Digestive System: They have a complete digestive system, typically in the form of a straight or U-shaped tube.
7. Gill Slits: Hemichordates may have dorso-lateral pharyngeal gill slits, ranging from one to several pairs. These slits serve as ciliary filter feeders.
8. Circulatory System: They possess a simple yet well-developed circulatory system. It is of the closed type and includes a dorsal heart, as well as two longitudinal vessels (dorsal and ventral) interconnected by lateral vessels and sinuses.
9. Excretory System: Excretion is carried out by a single glomerulus located in the proboscis.
10. Nervous System: Hemichordata has a relatively primitive nervous system consisting mainly of a subepidermal nerve plexus. The dorsal collar nerve cord is hollow.
11. Reproduction: Hemichordates primarily reproduce sexually, with separate sexes. They may have one to several pairs of gonads. Fertilization occurs externally in seawater. Development can be either direct or indirect, with the presence of a free-swimming tornaria larva in the latter case.

Some examples of Hemichordata include Balanoglossus, Saccoglossus, Rhabdopleura, Atubaria, and Dendrograptus.

In summary, Phylum Hemichordata comprises marine organisms with soft, worm-like bodies. They possess distinctive anatomical features, such as a proboscis, collar, and trunk. Their digestive, circulatory, and excretory systems are well-developed but relatively simple. Hemichordates exhibit a primitive nervous system, reproduce sexually, and may undergo indirect development with tornaria larvae.

Classification of Phylum Hemichordata 

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

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

Class 1- Enteropneusta, derived from the Greek words “enteron” meaning gut and “pneustos” meaning breathed, refers to a group of marine animals commonly known as “acorn” or “tongue worms.” Here are some key characteristics of this class:

1. Habitat and Lifestyle: Enteropneusts are solitary animals that can be either free-swimming or burrowing in marine environments.
2. Body Structure: They have elongated, vermiform bodies without a stalk. The body is divided into three main regions: the proboscis, collar, and trunk. Unlike some other hemichordates, the collar in Enteropneusta lacks tentaculated arms or a lophophore.
3. Alimentary Canal: Enteropneusts possess a straight alimentary canal, with the mouth and anus located at opposite ends. They are filter feeders, using their gill slits for feeding.
4. Gill Slits: They have numerous pairs of U-shaped gill slits, which aid in respiration and filter feeding.
5. Hepatic Caeca: Enteropneusta class typically has two pairs of hepatic caeca located in the middle of the trunk. These structures are involved in digestion and nutrient absorption.
6. Reproduction: Enteropneusts have separate sexes, with numerous sac-like gonads. They reproduce sexually, and their development can occur with or without a tornaria larval stage. Asexual reproduction is not observed in this class.

Some examples of Enteropneusta include Balanoglossus, Saccoglossus (also known as Dolichoglossus), Protoglossus, and Ptychodera.

In summary, Enteropneusta is a class within Phylum Hemichordata, consisting of marine acorn or tongue worms. They have worm-like bodies with distinct regions, a straight alimentary canal, and numerous pairs of gill slits. Enteropneusts exhibit separate sexes, and their reproductive development may involve a tornaria larva stage.

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

Class 2. Pterobranchia, derived from the Greek words “pteron” meaning feather and “branchion” meaning gill, refers to a group of marine animals with the following characteristics:

1. Habitat and Lifestyle: Pterobranchia are sedentary animals that can exist either in solitary or colonial forms. They are tubicolous, meaning they live inside secreted chitinous tubes.
2. Body Structure: They have short, compact bodies with a stalk-like structure for attachment to their tubes. The body is divided into three main regions: the proboscis, collar, and trunk. The proboscis possesses ciliated tentacles that generate ciliary feeding currents of water. The collar bears two or more tentaculated arms with tentacles.
3. Gill Slits: Pterobranchia typically have one pair of gill slits or may lack them entirely. These gill slits are not U-shaped like those found in other hemichordate classes.
4. Alimentary Canal: Their alimentary canal is U-shaped, with the dorsal anus situated near the mouth at the same end.
5. Reproduction: Pterobranchia exhibit separate or united sexes, with single or one pair of gonads. Their development is usually direct, although it may or may not include a free-swimming larval stage. Some species within this class also engage in asexual reproduction through budding.

Pterobranchia are sessile, tubicolous marine animals that use ciliated tentacles and a lophophore to create feeding currents. Their U-shaped alimentary canal, presence of tentaculated arms, and unique reproductive characteristics distinguish them within the phylum Hemichordata.

Order 1. Rhabdopleurida

Order 1. Rhabdopleurida is a taxonomic classification within the class Pterobranchia, and it includes a single genus called Rhabdopleura. Here are some key characteristics of this order:

1. Colonial Structure: Rhabdopleurida species are colonial, meaning they exist as a group of interconnected individuals called zooids. These zooids are connected by a stolon, which allows them to form a colony.
2. Collar with Tentaculated Arms: The collar region of Rhabdopleurida zooids possesses two tentaculated arms. These arms are specialized structures that aid in feeding and capturing food particles.
3. Absence of Gill Slits: Unlike other hemichordates, Rhabdopleurida species do not have gill slits. This distinguishes them from other members of the phylum.
4. Single Gonad: Each zooid in Rhabdopleurida has a single gonad, which is responsible for reproductive functions.

The only known genus within Order Rhabdopleurida is Rhabdopleura. It represents the sole example of this order and exhibits the aforementioned characteristics.

In summary, Order 1. Rhabdopleurida is a taxonomic grouping within the class Pterobranchia. It comprises colonial marine organisms connected by a stolon, with each individual having a collar bearing two tentaculated arms. These species lack gill slits and possess a single gonad. The single genus Rhabdopleura represents the only known example within this order.

Order 2. Cephalodiscida

Order 2. Cephalodiscida is a taxonomic classification within the class Pterobranchia, and it includes genera such as Cephalodiscus and Atubaria. Here are the key characteristics of this order:

1. Living Arrangement: Cephalodiscida species can exist as solitary individuals or as several zooids living in a common gelatinous case. While they may be physically close to one another, they are not interconnected like the colonial species in other orders.
2. Collar with Tentaculated Arms: The collar region of Cephalodiscida zooids possesses several tentaculated arms. These arms are equipped with tentacles and serve various functions, including feeding and capturing food particles.
3. Presence of a Single Pair of Gill Slits: Cephalodiscida species have a single pair of gill slits. These slits are specialized structures involved in respiration and water circulation.
4. Single Pair of Gonads: Each zooid in Cephalodiscida possesses a single pair of gonads, which are responsible for reproductive functions.

The examples of genera within Order Cephalodiscida include Cephalodiscus and Atubaria. These organisms exhibit the aforementioned characteristics and are representative of this order.

In summary, Order 2. Cephalodiscida is a taxonomic grouping within the class Pterobranchia. It comprises species that can exist either as solitary individuals or in groups within a shared gelatinous case. These organisms have a collar with several tentaculated arms, a single pair of gill slits, and a single pair of gonads. Genera such as Cephalodiscus and Atubaria exemplify the characteristics of this order.

Class 3. Planctosphaeroidea

Class 3. Planctosphaeroidea is a taxonomic classification within the phylum Hemichordata. This class is characterized by the presence of a few small, rounded, transparent, and pelagic larvae, believed to be specialized tornaria larvae of an unknown hemichordate species called Planctosphaera pelagica. Here are the key features of this class:

1. Larval Morphology: The larvae of Planctosphaeroidea are small, rounded, and transparent. They are pelagic, which means they inhabit the open water column rather than being associated with the seafloor. These larvae are thought to be specialized tornaria larvae, which are a distinctive larval form found in certain hemichordates.
2. Ciliated Bands: The body of the larvae is covered by extensively branched ciliary bands. These bands consist of rows of cilia, which are hair-like structures that beat in coordinated patterns to create water currents and facilitate locomotion and feeding.
3. U-Shaped Alimentary Canal: The larvae have an L-shaped alimentary canal. This means that the digestive tract forms a bent or U-shaped structure within the body. The alimentary canal is responsible for processing and digesting food.

It is important to note that while the larval form of Planctosphaeroidea is known, the adult form of this class is currently unknown. Further research is needed to understand the adult characteristics and life cycle of the hemichordate species associated with Planctosphaeroidea.

In summary, Class 3. Planctosphaeroidea is a taxonomic classification within the phylum Hemichordata. It comprises a small number of pelagic larvae, believed to be specialized tornaria larvae of an unidentified hemichordate species called Planctosphaera pelagica. These larvae have a rounded shape, are transparent, and possess extensively branched ciliary bands on their bodies. The larvae have an L-shaped alimentary canal. While the larval form is known, the adult characteristics of Planctosphaeroidea are currently unidentified.

Class 4. Graptolite

Class 4. Graptolite refers to a group of extinct colonial hemichordates that are primarily known from the fossilized remains of their tubes. These organisms were prevalent during the Ordovician and Silurian periods. Here are some key characteristics of this class:

1. Extinct Colonial Hemichordates: Graptolites are considered to be colonial hemichordates, which means they formed colonies consisting of multiple individuals. Each individual, or zooid, in the colony had its own specific function and role within the colony structure.
2. Fossilized Tubes: The main evidence for the existence of graptolites comes from the fossilized structures of their tubes. These tubes were composed of chitinous material and served as protective enclosures for the individual zooids. The fossilized remains of these tubes provide valuable insights into the morphology and distribution of graptolites.
3. Abundance in Ordovician and Silurian Periods: Graptolites were particularly abundant and diverse during the Ordovician and Silurian periods of the Paleozoic Era. They played a significant role in marine ecosystems during this time and left behind a rich fossil record.
4. Zooid Housing: Each graptolite animal resided within a zooid, which was an individual compartment within the colony. These zooids were interconnected within the colony structure and performed various functions necessary for the survival and reproduction of the graptolites.
5. Affinity with Rhabdopleura: Graptolites exhibit certain similarities in terms of their tubular chitinous skeleton and colonial habits with another hemichordate group called Rhabdopleura. This suggests a close evolutionary relationship between these two groups within the phylum Hemichordata.

Example:

An example of a graptolite is Dendrograptus. This genus is known from fossil records and represents one of the many species of graptolites that existed during the Ordovician and Silurian periods.

In summary, Class 4. Graptolite refers to an extinct group of colonial hemichordates. They are primarily known from the fossilized remains of their tubes, which served as protective enclosures for individual zooids. Graptolites were abundant during the Ordovician and Silurian periods and played a significant role in marine ecosystems. Each graptolite animal resided in a zooid within the colony structure. The affinity between graptolites and Rhabdopleura suggests a close evolutionary relationship. Dendrograptus is an example of a graptolite genus known from the fossil record.

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.

Representative Types

1. Saccoglossus (= Dolichoglossus)

Saccoglossus, also known as Dolichoglossus, is a typical enteropneust within the phylum Hemichordata. It shares many similarities in habits, habitat, and structure with Balanoglossus. Here are some key characteristics of Saccoglossus:

1. Habitat and Habits: Saccoglossus is a marine organism that inhabits spirally twisted furrows. It is a slender, soft-bodied tongue worm that resides in these marine environments.
2. Body Structure: The body of Saccoglossus is divisible into three main regions: proboscis, collar, and trunk. The proboscis, also known as the protosome, is exceptionally elongated and pointed compared to other tongue worms.
3. Collar and Trunk: The collar slightly overhangs the beginning of the trunk and covers the first three or four pairs of gill pores. The trunk is further differentiated into an anterior branchial region and a posterior branchial region. Saccoglossus lacks genital wings and hepatic caeca.
4. Alimentary Canal: The alimentary canal of Saccoglossus is straight, and the anus is located at the posterior end of the body. This straight digestive tract allows for efficient food processing and elimination.
5. Reproductive System: Saccoglossus exhibits separate sexes, with males and females having distinct reproductive organs. Mature gonads are yellow in males and gray in females. Dorsolateral genital folds externally mark the position of the gonads in the middle part of the trunk.
6. Gill-Slits: Saccoglossus possesses gill-slits, which are openings on the sides of the body for respiration and filter feeding. Tongue bars, which aid in filter feeding, hang freely in these gill-slits since synapticula, which typically support the bars, are absent.
7. Reproduction and Development: Fertilization in Saccoglossus is external, occurring in the marine environment. Unlike some other hemichordates, Saccoglossus undergoes direct development without a free-swimming tornaria larval stage.
8. Distribution: Saccoglossus is found in various regions, including the Indo-Pacific, Atlantic coasts, New Zealand, and Australia. It demonstrates a wide distribution within these marine habitats.

In summary, Saccoglossus, or Dolichoglossus, is a marine tongue worm within the enteropneust group of the phylum Hemichordata. It shares similarities with Balanoglossus in terms of habits, habitat, and body structure. Saccoglossus has an elongated proboscis, a collar that covers the gill pores, and a trunk differentiated into branchial regions. It exhibits separate sexes, has a straight alimentary canal, and lacks a free-swimming tornaria larval stage. Saccoglossus is distributed in various marine regions worldwide.

2. Ptychodera

Ptychodera is a marine organism that closely resembles Balanoglossus in terms of its habits, habitat, and overall structure. Here are the key characteristics of Ptychodera:

1. Habitat and Habits: Ptychodera is a soft-bodied, slender marine animal that burrows in the sand or mud. It is typically found in tropical and subtropical regions of the sea.
2. Body Structure: The body of Ptychodera is elongated and can be divided into three main parts: proboscis, collar, and trunk. The proboscis and collar are relatively short, while the trunk is elongated, contributing to the overall slender appearance of the organism.
3. Coloration: Ptychodera generally has a yellowish coloration, which may help it blend into its environment.
4. Gonads and Hepatic Caeca: The gonads of Ptychodera are located in the genital wings, which are specialized structures associated with reproduction. Distinct hepatic caeca, which are part of the digestive system, are also present.
5. Sexual Reproduction: Ptychodera exhibits separate sexes, with male and female individuals having distinct reproductive organs. This indicates a sexual mode of reproduction.
6. Larval Development: Ptychodera undergoes indirect development, which involves a free-swimming larval stage known as tornaria. The tornaria larva is characteristic of many hemichordates.
7. Bioluminescence: Ptychodera exhibits bioluminescence, which means it is capable of producing light. This ability may have various functions, such as attracting prey or mates, or as a defensive mechanism.
8. Geographic Distribution: Ptychodera is primarily found in the seas of tropical and subtropical regions, indicating a preference for warm waters.

In summary, Ptychodera is a soft-bodied marine organism that closely resembles Balanoglossus in its habits, habitat, and overall structure. It has an elongated body divided into proboscis, collar, and trunk, with distinct gonads and hepatic caeca. Ptychodera exhibits separate sexes, undergoes indirect development with a free-swimming tornaria larval stage, and displays bioluminescence. Its yellowish coloration and preference for tropical and subtropical regions contribute to its ecological characteristics.

3. Cephalodiscus

Cephalodiscus is a fascinating marine organism that forms large aggregations in tropical and temperate oceans. Here are the key characteristics of Cephalodiscus:

1. Habitat and Aggregation: Cephalodiscus individuals, also known as zooids, are found attached to objects on the sea bottom. They occur in large aggregations, but each zooid lives in a separate tube embedded in a common matrix called a coenoecium. Despite being in close proximity, the zooids are free from each other.
2. Body Structure: A zooid of Cephalodiscus shares a general structure similar to that of Balanoglossus. It consists of three main parts: proboscis, collar, and trunk. These parts are supported by a hollow muscular stalk.
3. Proboscis and Collar: The proboscis of Cephalodiscus is a ventrally bent disc, while the collar bears several hollow branched arms in two rows. These arms, known as tentaculated arms, are equipped with tentacles and cilia. The cilia create a current of water that carries food particles into the mouth.
4. Trunk and Alimentary Canal: The trunk of Cephalodiscus is a plump sac with a U-shaped alimentary canal. It has a ventral mouth and a dorsal anus. In the anterior part of the trunk, a single pair of gill-clefts can be found.
5. Reproductive System: Cephalodiscus exhibits separate sexes, with each individual having a pair of gonads. The gonads open through separate gonopores located dorsally on the front part of the trunk. Fertilization occurs internally, and Cephalodiscus undergoes direct development without a larval stage.
6. Coenoecium and Budding: The zooids of Cephalodiscus reproduce asexually through budding. On the trunk of the original individual, a peduncle develops, which bears buds. These buds eventually become free and form new zooids. Each zooid secretes its own tube within the coenoecium.
7. Distribution: Cephalodiscus is primarily found in tropical and temperate oceans, where it forms aggregations attached to objects on the sea bottom.

In summary, Cephalodiscus is a marine organism that forms aggregations in tropical and temperate oceans. Each zooid lives in a separate tube within a common matrix. It has a proboscis, collar, and trunk supported by a muscular stalk. The collar bears branched arms with tentacles for capturing food. The trunk has a U-shaped alimentary canal and a pair of gill-clefts. Cephalodiscus exhibits separate sexes with gonads and reproduces through budding within the coenoecium. It is primarily found in tropical and temperate oceans, where it creates fascinating and complex aggregations.

4. Rhabdopleura

Rhabdopleura is a fascinating colonial organism found primarily in the North Atlantic. Here are the key characteristics of Rhabdopleura:

1. Colonial Form: Rhabdopleura exhibits a true colonial form. The colonies are attached to stones, corals, and other sessile marine animals. The colony structure consists of a thin horizontal branching tube called a coenecium. From the coenecium, short erect tubes arise, each containing a zooid.
2. Zooid Structure: Each zooid in Rhabdopleura is minute in size and shares a general structure similar to that of an enteropneust. It consists of three main parts: proboscis, collar, and trunk. Attached to the trunk is a stalk. On the collar, a pair of hollow branching arms can be found. These arms possess cilia that aid in the collection of food for the zooid.
3. Alimentary Canal: The alimentary canal of Rhabdopleura zooids is U-shaped, with the anus situated close to the mouth. Unlike some other hemichordates, Rhabdopleura lacks gill-clefts, and a glomerulus is also absent.
4. Reproductive System: Rhabdopleura exhibits separate sexes, with each individual having a single gonad. The gonad has a gonopore located on the right side of the zooid. Large yolky eggs are laid as part of the reproductive process.
5. Budding and Tube Formation: Rhabdopleura reproduces asexually through budding. On the trunk of the original individual, a peduncle develops, which bears buds. However, unlike other species, these buds do not become free-swimming and instead remain attached, forming new zooids. Each zooid secretes its own ringed vertical tube or case, which is formed ring by ring. The horizontal part of each tube is connected by a black stolon, which links the various zooids to the parent zooid, resulting in the formation of a connected colony.
6. Distribution: Rhabdopleura colonies are primarily found in the North Atlantic, where they attach themselves to stones, corals, and other sessile marine animals.

In summary, Rhabdopleura is a colonial organism found in the North Atlantic. It forms colonies attached to various substrates using a branching coenecium. Each zooid within the colony has a proboscis, collar, and trunk, with a stalk attached. The zooids lack gill-clefts but possess branching arms for food collection. Reproduction occurs through budding, and each zooid secretes its own ringed vertical tube. Rhabdopleura exhibits separate sexes and produces large yolky eggs. It represents a unique and intriguing colonial form within the hemichordates.

FAQ

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.
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