Examples, Characteristics, And Classes Of Phylum Annelida

Have you ever found a common earthworm in your garden or the park? If you have, they will have belonged to the Phylum Annelida alongside a wide range of other body forms known as Annelids.

There are an estimated 22,000 species within the Phylum Annelida that can be categorized into four main classifications, Polychaeta, Oligochaeta, Hirudinea, and Archiannelida.

Annelids can be found in marine, terrestrial, and freshwater habitats, although certain conditions are required for them to survive in a terrestrial environment.  

In this article, we will explore in detail the characteristics, classifications, and examples of these creatures.

Examples

One of the easiest ways to gain an understanding of Phylum Annelida species is to look at examples of them.

We will look at an example of an Annelid from each of the habitats mentioned above to help create a knowledge base from which annelids can be explored further. 

Nereis

Nereis annelids are marine, nocturnal, carnivorous creatures. Common examples of these are sandworms and clamworms. You are most likely to find these annelids in burrows in areas of tidal activity along coastlines.  

The body of Nereis Annelida is formed of three divided sections, the head, the trunk, and the pygidium (shield). Their body possesses setae (bristles) and parapodia (appendages) to facilitate movement along the floor of the ocean, as well as gas exchange.

There are two types of setae in Nereis annelids. Acicular setae provide support to the annelid to allow for accurate movement. Locomotor setae assist the annelid in the crawling motion utilized to travel across the ocean floor.

The fertilization of Nereis Annelida takes place in seawater.

Pheretima

Pheretima annelids are cylindrical, soft-bodied, and long-segmented annelids that live in burrows in the soil, or terrestrial habitats.  They are commonly known as earthworms.

Pheretima annelids exclusively feed and reproduce at night, making them nocturnal creatures and earning them the nickname Nightcrawlers. Pheretima annelids are herbivores, feeding mostly on fallen leaves and other plant debris.

They are hermaphrodites, meaning that they have both male and female sexual organs.  

Hirudinaria

Hirudinaria annelids are also known as the Indian Cattle Leech. These annelids are commonly found in freshwater habitats such as ponds, lakes, and slow streams.

They are ectoparasitic, meaning that they feed on the blood of fish, frogs, and humans, and they are blood-sucking by nature. The mouth of a Hirudinaria annelid is triradiate and is guarded by velum as is common with certain invertebrates.  

The body of Hirudinaria annelids are soft, vermiform (worm-like), and segmented. The body of a leech is divided metamerically into 33 segments and features anterior and posterior suckers.

The anterior sucker is to facilitate adhesion, and the posterior sucker is to assist in locomotion. 

Characteristics

There are certain characteristic features that annelids exhibit that can be useful in identifying them.  The characteristics of annelids pertain to their physiology and habitats.

In this article, we will be discussing the characteristics of annelids by their classification rather than specific species due to the fact that their characteristics span multiple species present in single classifications. 

Reproduction And Life Cycle

There are some annelids that are capable of both sexual and asexual reproduction, however, there are many that are only capable of a single mode of reproduction.  

Sexual Reproduction Of Polychaete And Oligochaete

There is one main difference in the ways that Polychaeta and Oligochaeta reproduce.  The majority of Polychaete annelids are dioecious organisms which means that they either possess male or female reproductive organs.

In contrast to this, Oligochaete annelids are hermaphrodites which means that they each possess both the male and female reproductive organs.  

Oligochaete annelids require a mating process to take place in order to reproduce.  Because they are simultaneous hermaphrodites, these annelids are able to pass their sperm to their mating partner while also receiving sperm to fertilize their own eggs.

During the mating process, the sperm is deposited into the spermatheca which is a sac-like structure, where it will be stored until the eggs are ready to be fertilized.

A cocoon will be secreted by the non-segmented, thickened glandular section of the body near the head, called the clitellum.  The eggs and the sperm will then be deposited into the cocoon for fertilization to take place.  

The embryo grows within the cocoon for the full gestation period, after which the young Oligochaete annelid will emerge, forgoing the larval stage. This is known as direct development. 

The sexual reproduction of Polychaete annelids differs from that of Oligochaete annelids in that there is no direct mating involved.

Instead, some Polychaete annelids have been shown to undergo a physical transformation during times of sexual reproduction.

Specifically, pre-existing structures on the posterior of the creatures transform into either the male or female reproductive organ that is capable of releasing gametes or reproductive cells such as eggs or sperm.

The majority of Polychaete annelids already possess either male or female reproductive organs.

The process of fertilization takes place externally from the body of Polychaete annelids. Rather, the organisms will swim to the surface of the water where they will release their gametes, which will result in external fertilization.

The resulting fertilized eggs usually hatch into trochophore larvae, these larvae will float among the plankton in the water and eventually, will metamorphose into their adult form by gradually adding segments to their body. 

Asexual Reproduction Of Polychaete And Oligochaete

Both Polychaete and Oligochaete annelids are also capable of asexually reproducing. There are two mechanisms by which these annelids can perform asexual reproduction called fission and budding.

These mechanisms take place without the need for the exchange of gametes.

In the case of asexual fission reproduction, the organism will split its body into two.  Both parts will then regenerate the respective missing part in order to become a complete organism.

This specific form of reproduction is particularly prevalent in worm species with elevated regenerative capacity.  

Alternatively, the process of budding as a form of asexual reproduction involves an adult organism developing a bud-like growth on the outside of its body.

Over time, the bud will develop into a new, individual organism that is similar to the host or parent organism. Once the new organism has fully developed, it will break off from the parent and will be capable of reproducing in the same way.

Sexual Reproduction Of Hirudinea

Hirudinea annelids are exclusively capable of reproducing sexually. However, similarly to the Oligochaete annelids, Hirudinea annelids are hermaphroditic, possessing both male and female reproductive organs.

Most commonly a Hirudinea organism will possess several pairs of testes and one pair of ovaries, all of which share a single ventral gonopore, or genital opening. 

As with all sexually reproducing organisms, mating is required for Hirudinea annelids to produce young. Central to the mating process is the clitellum which is only present during the spring mating season.

The clitellum will produce mucus during mating that helps keep the pair locked together throughout the process. The sperm is deposited into the spermatheca for fertilization.

In Hirudinea annelids, the fertilization of the eggs takes place within the body of the organism before they are released into the cocoon.  As with Oligochaete annelids, the Hirudinea annelids skip the larvae stage and engage in direct development. 

Respiratory And Circulatory systems

All of the species in the Phylum Annelida possess a closed circulatory system, which means that the organism’s blood is contained within vessels at all times.

There are generally only two primary vessels within the circulatory system, known as the ventral and dorsal vessels.

The former transports blood from the head to the tail of the organism, the latter does the opposite and transports blood from the tail back up to the head.

The movement of blood in Annelids is either facilitated by muscular actions or a heart-like vessel located in the anterior body segment.

Contained within the two body segments of annelids are two smaller vessels in addition to the aforementioned ones. These two smaller vessels connect the main vessels to the copious capillaries that permeate the organism’s epidermis.

These capillaries are crucial in the process of respiration as gaseous exchange takes place via the skin.

In marine and freshwater habitats, as well as terrestrial habitats, Annelids absorb oxygen from the environment through the capillaries in their skin which is then transported by the blood vessels.

When the blood vessels return with carbon dioxide the gas is then diffused through the same capillaries to remove it from the organism’s body.  In some annelids in marine or freshwater habitats, specialized gills may be used for respiration.

Nervous System

The species within the Phylum Annelida have been studied by scientists for many years. Within the studies that have been completed, evidence shows that annelids exhibit a vast array of variations in their nervous systems.

However, the majority of annelid species have a simple brain, a pharynx, ganglions, and sensory nerves.

The nervous system within annelids forms a small, brain-like structure near the anterior end of the organism just in front of the pharynx.

This structure is a concentration of nerve endings and tissue designed to control physiological functioning. Studies have shown that the brain structure is bi-lobed and the sensory nerves that originate from it extend throughout the length of the organism. 

Within each segment of the annelid’s body, there is a swelling or collection of nerve endings known as a ganglion. Each ganglion leaves 2 to 5 nerve pairs to feed the subsequent segments. Annelids also feature sensory organs such as eyes, palps, and antennae.

The nervous system of marine annelids differs slightly from the above mentioned nervous system. Studies have shown that annelids that reside in marine environments have a brain-like structure that features three sections: fore, mid, and hindbrain. 

The function of the nervous system in annelids is primarily to facilitate movement. Most land-based annelids display well-coordinated compression and relaxation of segment muscles as their means of locomotion.

The nervous system also allows annelids to detect threats and react accordingly.  They can do this by increasing their speed or by utilizing drastic contractions of their muscles to produce an evasive response.

Digestive System

Annelids reside in various habitats and therefore have access to different types of food depending on their environment and as a result, have different digestive systems. 

Oligochaete annelids such as earthworms are known as deposit feeders. This means that they obtain the nutrients that they require by ingesting some of the soil that they burrow through it.

Due to the way in which Oligochaete annelids come by their nutrients, organisms such as earthworms feature gizzards to assist in the breaking down of the soil.

As a result of their ability to break down plant debris such as leaves and grass, it is common for them to be used within the agricultural industry and in the gardening community to help improve the condition and fertility of the soil.  

Polychaete annelids that reside in marine habitats have a different system by which they obtain nutrients.

These annelids acquire the nutrients necessary to sustain them by either preying on other organisms, scavenging by feeding on dead organisms or through a filter method of eating suspended food material in the water.  

Lastly, Hirudinea annelids rely entirely on other organisms to procure the nutrients necessary for their survival, therefore they are classed as a parasite.

Because of this, Hirudinea annelids such as leeches have a proboscis, or tubular sucking organ, as well as jaws and teeth.  This helps facilitate the breaking of skin in order to suck the blood from its host.

Additionally, leeches produce an anticoagulant and analgesic agents in their saliva. This prevents the host from feeling pain and helps maintain the flow of blood for the annelid.

Despite these differences in food sources and habitats, all annelids have complete digestive tracts consisting of a mouth, pharynx, esophagus, stomach, intestines, and anus.

Food material travels through the entire digestive system, nutrients are extracted and absorbed, then waste material is excreted via the anus. 

Habitats

Annelids are commonly present in three types of habitat.  Marine and freshwater habitats make up about 81% of the annelid population, and high humidity terrestrial habitats such as damp earth account for about 12% of the annelid population.

Annelids can be found all over the world.

Marine

Annelids that populate marine habitats are members of the Polychaete classification and include examples such as the Pompeii worm ( Alvinella Pompejana ) that is endemic to the hydrothermal vents located in the Pacific Ocean.  

Marine Annelids feature parapodia that assist in gas exchange and the facilitation of locomotion, both swimming and crawling, as mentioned above.

The setae or bristles that are present on the parapodia provide protection for the annelid from predators. The main way in which they do this is to make the Annelid difficult or uncomfortable for the prey to swallow.

Some species of Annelid also secrete poison from the parapodia and setae when they are under threat to protect themselves. 

Freshwater

Annelids that stem from the Hirudinea classification, such as leeches, can most often be located in a freshwater habitat, although they are also capable of surviving in a terrestrial habitat.  

The Annelids that reside in freshwater habitats have developed particular adaptations that allow them to survive in such environments.

These organisms, despite having five pairs of eyes, have very poor eyesight which only allows them to differentiate between light and dark areas.

However, this is beneficial to their survival as it allows them to detect their location (near the surface or in the depths) and allows them to detect the presence of predators through shadows.  

Another adaptation that these leeches have to help them survive is the existence of chemoreceptors that allow them to detect changes in the chemicals present in the water that can inform them of the presence and directional location of predators. 

Finally, these organisms have developed methods of moving both through the water and on land.

When they are in the water they can either perform a swimming action that is achieved via creating a wave-like motion through the contraction of their muscles, or they can travel using a method known as looping.

Looping involves fixing the anterior sucker to a surface, the organism then contracts its muscles to pull its body forward, bringing the posterior part of its body to the front.

The posterior sucker is then attached to the surface and the process is repeated.  

Terrestrial

Oligochaete annelids are often found in terrestrial habitats, the most common of which is the earthworm.  The most conventional type of terrestrial habitat contains a high level of moisture with access to an abundance of organic material to ingest.  

Oligochaete annelids such as earthworms do not have eyes due to the amount of time that is spent in subterranean conditions.  Instead, they use other sensory structures present in their nervous system such as palps to feel their way around.

These organisms often ingest the soil as they burrow through it in a grazing manner.  When they have absorbed the necessary nutrients from the ingested material they secrete the waste through mucus.  

Within the terrestrial habitat, organisms such as the earthworm create movement through the contraction and relaxation of their segments.

Classifications

All annelids in the Phylum Annelida are bilaterally symmetrical triploblastic organisms with a segmented body. However, there are several differences between organisms within the four classes. 

Below we will detail the main differences that help to classify annelids.

Polychaeta

The term Polychaeta is made from the Greek words for “many” and “bristles”.  This describes the physical characteristics of the members of this classification accurately.

The Polychaete classification is the most populous of the four within the Phylum Annelida, comprising over 10,000 different species located within the marine and freshwater aquatic habitats.  

The annelids in this classification feature a head region that consists of sensory structures such as antennae and palps, as well as a peristomal body segment that houses the mouth of the organism and the olfactory sensory structure. 

The main body segment of Polychaete annelids consists of essential internal and external organs and features such as the parapodia and setae that are the main characteristics of these organisms and vital to the creatures’ movement, gaseous exchange mechanism, and survival.  

Oligochaeta

The term Oligochaeta in Modern Latin means “few bristles” because these annelids have fewer setae and parapodia than the Polychaeta classification does.

In comparison with the Polychaeta and Hirudinea classifications, the Oligochaeta group is considered to be the most diverse and abundant with over 3,000 different species spread across multiple habitats. 

Dissimilar from the Polychaete annelids, there is no clearly defined head in these organisms, making it difficult to determine the anterior and posterior ends of the creature.

The anterior end can be established when necessary by locating the clitellum which is zonally closer to the anterior end.  

The body of Oligochaete annelids contains between 100 and 150 segments.  Microscopic setae are present on each segment that help to provide stability and support during movement.

The motion of movement displayed by Oligochaete annelids is created through the coordinated contraction and relaxation of muscles along the length of the body of the organism.  

Hirudinea

The Hirudinea classification within Phylum Annelida pertains to true leeches.

The organisms in this classification have the soft, cylindrical bodies characteristic of all annelids, but are much shorter than the previous classifications with only around 30 to 35 segments per organism.  

While they feature multiple sets of eyes, their vision is poor with only the ability to detect light and dark.

This suits their more common aquatic freshwater habitat, allowing them to orientate themselves with either the surface or the bottom of the water. The detection of shadows also allows them to locate predators.  

The Hirudinea annelids feature anterior and posterior suction mechanisms that assist in feeding and locomotion.

Unlike the Oligochaete and Polychaete classifications, the true leeches have jaws and teeth that are powerful enough to break through the skin of their host, as well as anti-clotting and analgesic agents in their saliva to facilitate their parasitic feeding method.

Archiannelida

The Archiannelida classification of annelids is the smallest of the four. The annelids in this classification are very simple, featuring an elongated body, however, the segmentation is very shallow and they do not appear to have any parapodia or setae. 

Economic Importance Of Phylum Annelida

All four classifications within the Phylum Annelida provide a level of economic importance.  

The leeches featured in the Hirudinea class have been used to develop medicines that have created vast advancements in medical science that may not have been possible without the studies carried out focusing on them.

The earthworms from the Oligochaeta class are widely considered to be the agricultural industry’s best friend due to their natural ability and desire to improve the quality and fertility of the soil.

Annelids from all four classifications are often used for dissection within zoological laboratories as they are easy to obtain, this allows for the further understanding of ecosystems, water quality, soil quality, and our effect on the environment.

Finally, organisms such as annelids provide a plentiful, easily accessible food source for many other animals such as frogs, lizards, birds, and snakes, helping to sustain wider food chains and ecosystems. 

Final Thoughts

There are many examples of annelids within the four classifications that make up the Phylum Annelida.

Each organism within these classifications plays an important role in the ecosystem of its habitat and has adapted to handle its environment with optimum reproductive and digestive systems. 

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