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Phylum Annelida – Definition, Characteristics, Classification, Examples

Lamarck, 1809
Phylum Annelida
Phylum Annelida |, Public domain, via Wikimedia Commons
  • Phylum Annelida is a diverse group of organisms within the animal kingdom. They are commonly known as segmented worms or annelids, and they exhibit a wide range of adaptations and lifestyles. Annelids can be found in various environments, including aquatic habitats such as marine and freshwater ecosystems, as well as terrestrial habitats with moist conditions.
  • Annelids are characterized by their segmented bodies, which sets them apart from other organisms. The term “annelid” comes from the Latin word “anellus,” meaning “little ring,” referring to the segmented structure of their bodies. These organisms are bilaterally symmetrical, meaning their left and right sides are mirror images of each other. They are triploblastic, which means they have three primary germ layers during embryonic development, and they possess a coelom, a fluid-filled body cavity lined with mesoderm.
  • One notable feature of annelids is the presence of parapodia, which are appendages used for locomotion in many species. These structures aid in swimming, crawling, or burrowing through their respective habitats. The traditional classification of annelids divided them into three major groups: polychaetes, oligochaetes, and leeches.
  • Polychaetes are predominantly marine worms, while oligochaetes include earthworms, and leeches are typically associated with freshwater environments. However, recent cladistic research has challenged this classification, suggesting that leeches are a sub-group of oligochaetes, and oligochaetes are a sub-group of polychaetes.
  • Additionally, other organisms previously considered separate phyla, such as Pogonophora, Echiura, and Sipuncula, are now regarded as sub-groups of polychaetes. Annelids belong to the Lophotrochozoa, a super-phylum that includes mollusks, brachiopods, and nemerteans.
  • The basic body plan of an annelid consists of multiple segments, with each segment containing the same sets of organs. In most polychaetes, each segment also possesses a pair of parapodia. However, some annelids, like the Echiura and Sipuncula, do not exhibit obvious signs of segmentation. In species with well-developed septa, which are internal walls that separate segments, the circulatory system operates solely within blood vessels.
  • The segments near the front end of these species often have muscular vessels acting as hearts. The presence of septa enables annelids to change the shape of individual segments, facilitating movement through peristalsis (rippling motion) or undulations that enhance the effectiveness of the parapodia.
  • In species without septa, the blood circulates through the main body cavity without a specialized pump. Consequently, annelids employ a wide range of locomotory techniques, with some burrowing species reversing their pharynges to drag themselves through sediment.
  • Annelids play crucial roles in ecosystems. Earthworms, which are oligochaetes, contribute to terrestrial food chains as both prey and soil enrichers. Marine polychaetes, which can constitute a significant portion of species in near-shore environments, promote the development of ecosystems through their burrowing activities.
  • Their burrows facilitate the penetration of water and oxygen into the sea floor. Annelids also serve as food sources and bait for humans. Scientists study annelids to monitor the quality of marine and freshwater environments.
  • Although blood-letting is less common in medical practice today, some leech species are considered endangered due to over-harvesting for this purpose over the past few centuries. Furthermore, engineers are studying the jaws of ragworms, as they possess a remarkable combination of lightness and strength.
  • Due to their soft-bodied nature, annelid fossils are relatively rare. Fossils of annelids primarily consist of jaws and mineralized tubes secreted by some species. While some late Ediacaran fossils may represent annelids, the oldest fossil confidently identified as an annelid dates back to approximately 518 million years ago in the early Cambrian period.
  • Fossils of most modern mobile polychaete groups appear by the end of the Carboniferous period, around 299 million years ago. Paleontologists have debated whether certain body fossils from the mid-Ordovician period, approximately 472 to 461 million years ago, belong to oligochaetes. The earliest indisputable fossils of the annelid group appear in the Paleogene period, which began 66 million years ago.
  • In summary, phylum Annelida encompasses a diverse array of segmented worms found in marine, terrestrial, and freshwater habitats. These animals exhibit metamerism, or true segmentation, and display a range of adaptations for locomotion and survival.
  • Annelids contribute to ecosystem dynamics, serve as food and bait, and have both ecological and medical significance. While their fossils are rare, they provide valuable insights into the evolutionary history of these remarkable organisms.

Morphology of Annelida

  • The morphology of Annelida is characterized by its worm-like appearance and bilateral symmetry. The body of an annelid is divided into multiple segments, exhibiting a segmented body plan known as metamerism. This segmentation allows for the repetition of internal and external morphological features in each segment. Metamerism provides several advantages, such as increased size by adding compartments and more efficient movement.
  • The development of metamerism in annelids is believed to stem from identical teloblast cells during the embryonic stage. These cells give rise to identical mesodermal structures, contributing to the repeated segments throughout the body.
  • The overall body of an annelid can be divided into three main regions: the head, body, and pygidium (tail). The head region is located at the anterior end of the animal and contains sensory organs and specialized structures for feeding. The body region consists of the series of segments extending from the head to the pygidium. Each segment typically contains a set of organs and structures that are repeated in a coordinated manner. The pygidium is found at the posterior end of the animal and is involved in waste elimination.
  • A notable reproductive structure in annelids is the clitellum, which is responsible for the production of mucus. This mucus aids in the transfer of sperm between individuals during reproduction. Additionally, the clitellum plays a crucial role in the formation of a cocoon within which fertilization takes place. The clitellum appears as a fused band in the anterior third of the annelid’s body.
  • Overall, the morphology of Annelida is characterized by its segmented body, metamerism, and the presence of specialized structures such as the clitellum that facilitate reproduction and ensure the survival of the species.
Internal anatomy of a segment of an annelid
Internal anatomy of a segment of an annelid | Image Source:

Anatomy of Annelida

This schematic drawing shows the basic anatomy of annelids in a cross-sectional view.
This schematic drawing shows the basic anatomy of annelids in a cross-sectional view. | Image Source:
  • The anatomy of Annelida exhibits several distinctive features. The epidermis, which is the outermost layer of the body, is protected by a thin, acellular cuticle. This cuticle is not as thick as the cuticle found in ecdysozoans and does not require periodic shedding for growth. Beneath the epidermis, both circular and longitudinal muscles are present, contributing to the annelid’s movement.
  • One notable anatomical feature found in every segment of annelids is the presence of setae or chaetae. These are chitinous hairlike extensions that originate from the epidermis and project from the cuticle. Setae serve various functions such as providing traction for movement and anchoring the worm within its environment.
  • Annelids possess a true coelom, which is a body cavity derived from the embryonic mesoderm. This coelom is a significant advancement compared to other worms. Within the coelom, the body segments are divided by membranous septa, creating a series of compartments.
  • The digestive system of annelids, particularly in earthworms (oligochaetes), is well-developed and complete. It includes a mouth, muscular pharynx, esophagus, crop, gizzard, intestine, and anal opening. The gizzard aids in mechanical digestion, while the intestine is responsible for nutrient absorption and waste elimination.
  • Annelids have a closed circulatory system, consisting of dorsal and ventral blood vessels that run parallel to the alimentary canal. Capillaries provide circulation to individual tissues, and transverse loops connect the vessels in each segment.
  • Respiration in annelids occurs through the moist body surface since they lack a well-developed respiratory system. Gas exchange takes place across the body surface, which needs to remain moist for efficient respiration.
  • Excretion is facilitated by metanephridia, which are paired excretory organs present in each segment on the ventral side. Metanephridia consist of convoluted tubules and open, ciliated funnels, resembling primitive “kidneys.” They play a role in waste excretion and osmoregulation.
  • Annelids exhibit a well-developed nervous system. Around the pharynx, a nerve ring composed of fused ganglia is present. The nerve cord runs ventrally along the body and contains enlarged nodes or ganglia in each segment, allowing for coordination of sensory and motor functions.
  • Reproduction in annelids can be monoecious, with permanent gonads, as seen in earthworms and leeches, or dioecious, with temporary or seasonal gonads, as seen in polychaetes. Hermaphroditic annelids prefer cross-fertilization, but they can also engage in simultaneous hermaphroditism and exchange sperm during copulation.
  • Overall, the anatomy of Annelida showcases specialized structures and systems such as the coelom, digestive system, circulatory system, excretory system, and nervous system that contribute to their diverse and adaptive characteristics.

General Characters of Phylum Annelida

Phylum Annelida exhibits several general characteristics that define the group:

  1. Habitat: Annelids are found in various habitats, including marine, freshwater, and terrestrial environments. They can be free-living or parasitic, depending on the species.
  2. Symmetry: Annelids are bilaterally symmetrical, meaning their bodies can be divided into two equal halves through a single plane passing through the center.
  3. Germ layer organization: Annelids are triploblastic organisms, possessing three germ layers during embryonic development. These layers include the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer).
  4. Body cavity: Annelids are coelomates, having a true coelom, which is a body cavity lined by mesoderm or coelomic epithelium. The coelom is present between the body wall and the gut wall and is divided into compartments by internal partitions.
  5. Segmentation: Annelids exhibit segmentation, with their bodies externally and internally divided into segments or metameres. The segments are separated externally by ring-like grooves called annuli, and internally by transverse septa. Most segments contain a serial repetition of organs.
  6. Locomotion: Annelids employ various means of locomotion. Muscular contractions of longitudinal and circular muscles enable movement. Some aquatic annelids have specialized unjointed appendages called parapodia, which aid in swimming. Earthworms have bristle-like structures called setae on each body segment that assist in preventing backward slipping.
  7. Organ systems: Annelids have well-developed organ systems, including the digestive, circulatory, nervous, and excretory systems.
    • Digestive system: Annelids possess a complete digestive system with separate openings for ingestion (mouth) and egestion (anus).
    • Circulatory system: Annelids have a closed circulatory system, with blood circulating through enclosed spaces such as blood vessels.
    • Nervous system: Annelids have a nervous system consisting of ganglia in pairs, connected to a double nerve cord on the ventral side of the body through lateral nerves.
    • Excretory system: Annelids have specialized excretory organs called nephridia, composed of coiled tubular structures. Nephridia aid in osmoregulation and excretion of waste products.
  8. Other characteristics: Annelids respire through their body surface, and some species have gills or parapodia for respiration. They exhibit cephalization (the concentration of sensory and nervous structures at the anterior end), and many species can regenerate lost body parts. Annelids can be hermaphroditic, with both male and female reproductive organs present in the same individual. They reproduce both sexually and asexually, depending on the species.

Some examples of annelids include earthworms and leeches.

Classification of Annelida

On the basis of the presence or absence of parapodia, setae, metameres, and other morphological characteristics, approximately 8,700 known species of Annelida are divided into four major classes.

Class 1- Polychaeta (Gr., poly=many, chaeta=bristles/hair)

  • Polychaeta is a class of annelid worms, primarily found in marine environments, although some species can be found in freshwater habitats as well. They are carnivorous organisms, feeding on a variety of small invertebrates and sometimes even larger prey. Polychaetes exhibit both internal and external body segmentation.
  • The head region of Polychaeta consists of two parts: the prostomium and peristomium. This region bears various sensory organs, including eyes, tentacles, cirri, and palps. Lateral parapodia, which are appendages on the sides of the body segments, are adorned with numerous bristles called setae. Unlike other annelids, Polychaeta lacks a clitellum, which is a specialized reproductive structure found in earthworms and leeches.
  • Respiration in Polychaeta is facilitated by either cirri, branchiae (gills), or both. These structures enable the exchange of gases with the surrounding environment. The coelom, the body cavity, is spacious and typically divided by intersegmental septa, which provide structural support and maintain the internal organization of the organism.
  • The alimentary canal of Polychaeta is equipped with an eversible buccal region and a protrusible pharynx. This adaptation allows them to capture and consume their prey effectively. The excretory organ in Polychaeta is a pair of segmentally arranged nephridia, which help eliminate waste products from the body.
  • Polychaeta exhibits separate sexes, with temporary gonads present in many segments. Fertilization occurs externally, where eggs and sperm are released into the water, allowing for the union of gametes. Additionally, some species of Polychaeta are capable of asexual reproduction through lateral budding, where new individuals form as outgrowths from the parent organism.
  • During their development, Polychaeta go through a trochophore larval stage, which is characteristic of many marine invertebrates. This larval stage aids in dispersal and settlement of the species.
  • Polychaeta is further divided into two subclasses: Errantia and Sedentaria.
  • Eg., Pheretima, Tubifex

Subclass 1. Errantia

  • The Errantia subclass includes polychaetes that exhibit free-swimming, crawling, burrowing, or tube-dwelling behaviors. Their body segmentation is similar, except at the anterior and posterior ends. The prostomium is distinct and bears sensory organs. The parapodia, which are well-developed throughout the body, possess cirri. The pharynx is protrusible, enlarged, and often equipped with jaws and teeth.
  • Examples of polychaetes in this subclass include Nereis, Aphrodite, Polynoe, Phyllodoce, Tomopteris, Syllis, Eunice, and Histriobdella.

Subclass 2. Sedentaraia

  • The Sedentaria subclass comprises polychaetes that primarily exhibit burrowing and tube-dwelling habits. Their bodies are divided into two or more regions, with unlike segments and parapodia. The head region is small or highly modified, lacking eyes and tentacles, and with a reduced prostomium. They do not possess acicula (bristle-supporting structures) or compound setae. The pharynx is non-protrusible and lacks jaws and teeth. When present, gills are localized to the anterior segments. Sedentaria polychaetes primarily feed on plankton or organic detritus.
  • Examples of polychaetes in this subclass include Chaetopterus, Arenicola, Owenia, Sabella, Terebella, Sabellaria, and Pomatocerous.

It is important to note that the division of Polychaeta into the Errantia and Sedentaria subclasses, as described by Fauvel and Dab, is considered artificial and not a natural classification. The classification of Polychaeta is a subject of ongoing scientific investigation and refinement.

Class 2- Oligochaeta (Gr., oligos=few+ chaete=hair)

  • Oligochaeta is a class of annelid worms that primarily inhabit terrestrial environments, although some species can be found in freshwater habitats as well. Unlike Polychaeta, which have numerous bristles or setae, Oligochaeta have only a few setae embedded in their skin. They exhibit conspicuous external and internal segmentation throughout their body.
  • The head region of Oligochaeta is indistinct and lacks sensory organs. Instead, they rely on touch and chemoreceptors located throughout their body. Unlike Polychaeta, Oligochaeta lack parapodia, which are appendages found on the sides of the body segments.
  • One prominent feature of Oligochaeta is the presence of a glandular clitellum, which is a thickened band located in a specific region of the body. The clitellum secretes a mucus-like substance that plays a vital role in cocoon formation during reproduction.
  • The pharynx of Oligochaeta is not eversible and lacks jaws. Instead, it aids in the ingestion and digestion of organic matter through muscular contractions. Oligochaeta are hermaphroditic, meaning they possess both male and female reproductive organs. The testes are situated anterior to the ovaries.
  • The development of Oligochaeta is direct, without any larval stage. Fertilization occurs externally within the cocoon, which is produced by the clitellum. Oligochaeta exhibit two orders: Archioligochaeta and Neooligochaeta.

Order 1. Archioligochaeta

  • Archioligochaeta primarily consists of freshwater forms. They have a body consisting of a few segments and possess setae in bundles. The gizzard, a muscular organ involved in grinding food, is poorly developed or absent. The clitellum in Archioligochaeta is simpler, consisting of a single layer of cells and located far towards the posterior end. Eyespots are frequently present, providing light-sensitive capabilities. The male reproductive openings are positioned in front of the female reproductive openings. Reproduction in Archioligochaeta can occur through both asexual and sexual means.
  • Examples of Archioligochaeta include Tubifex and Aelosoma.

Order 2. Neooligochaeta

  • Neooligochaeta, on the other hand, mainly comprises terrestrial forms. They have a larger body with numerous segments. The arrangement of setae follows a lumbricine pattern. The gizzard is well developed, assisting in food processing. The clitellum in Neooligochaeta is composed of two or more layers of cells and begins after the twelfth segment. The female genital aperture is consistently located on the 14th segment, while the male pore lies a few segments behind. Vasa differentia, the ducts that transport gametes, are elongated and extend over three or four segments. Unlike Archioligochaeta, Neooligochaeta lacks eyespots. Reproduction in Neooligochaeta is solely sexual, and asexual reproduction is not known to occur.
  • Examples of Neooligochaeta include Pheretima, Eutypheus, Megascolex, and Lumbricus.

Oligochaeta play important ecological roles in soil fertility and nutrient cycling, contributing to the health of terrestrial ecosystems.

Class 3- Hirudinea (L., hirudo= a leech)

  • Hirudinea, commonly known as leeches, belong to the class Hirudinea. They exhibit various lifestyles, including ectoparasitic, blood-sucking, and carnivorous habits. While most leeches are aquatic, some species can be found in marine, freshwater, or even terrestrial environments.
  • The body of Hirudinea is elongated and usually flattened in a dorso-ventral or cylindrical manner. It consists of a fixed number of segments, typically 33. Each segment is further divided into 2 to 4 rings or annuli. Unlike other annelids, the segmentation in leeches is external, without the presence of internal septa. They lack parapodia and setae, which are characteristic features of other annelid groups.
  • Both the anterior and posterior ends of the leech’s body possess ventrally situated suckers. These suckers aid in attachment to hosts or substrates. The mouth is located on the ventral surface of the anterior sucker, while the anus opens dorsally to the posterior sucker.
  • The coelom, the body cavity, in leeches is greatly reduced due to the filling of botryoidal tissues, resulting in the formation of haemocoelomic sinuses. This reduction in the coelom is an adaptation to their parasitic lifestyle.
  • Leeches are hermaphroditic, possessing both male and female reproductive organs. They have one male and one female gonopore for reproduction. Fertilization occurs internally, with the transfer of sperm from one leech to another. Asexual reproduction is not known to occur in leeches. Eggs are laid in cocoons, which provide protection and support for the developing embryos. The development of leeches is direct, without a free-swimming larval stage.
  • The class Hirudinea is further divided into four orders: Acanthobdellida, Rhynchobdellida, Gnathobdellia, and Pharyngobdellida.
  • Eg., Hirudinaria

Order 1. Acanthobdellida

Acanthobdellida consists of leeches primarily parasitic on the fins of salmon fishes. They have a body comprising only 30 segments. These leeches are considered primitive, lacking anterior suckers, a proboscis, and jaws. Double rows of setae are present in the first five anterior segments. The body cavity is spacious but incompletely divided by septa. The vascular system consists of dorsal and ventral vessels. Nephridial openings are situated on the surface between the segments. The Acanthobdellida order includes a single genus and species, Acanthobdella, which is parasitic on salmon.

Order 2. Rhynchobdellida

  • Rhynchobdellida includes leeches that parasitize snails, frogs, fishes, and can be found in marine and freshwater environments. Each typical body segment consists of 3, 6, or 12 rings. The mouth is a small median aperture situated in the anterior suckers. Rhynchobdellida leeches possess a protrusible proboscis but lack jaws. Their coelom does not have compartments, and the blood vascular system is separate from the coelomic sinuses. The blood of these leeches is colorless.
  • Examples of Rhynchobdellida leeches include Placobdella, Helobdella, Piscicola, and Branchellion.

Order 3. Gnathobdellia

  • Gnathobdellia encompasses freshwater and terrestrial leeches that are ectoparasitic and blood-sucking in nature. Each typical body segment consists of 5 rings or annuli. The anterior suckers of Gnathobdellia leeches possess three jaws: one median dorsal and two ventrolateral. They do not have a proboscis, and their blood is red-colored. Botryoidal tissues are present in these leeches.
  • Examples of Gnathobdellia leeches include Hirudo, Hirudinaria, Haemadipsa, and Herpobdella.

Order 4. Pharyngobdellida

  • Pharyngobdellida comprises leeches found in both terrestrial and aquatic environments, with some species exhibiting a predaceous lifestyle. Their pharynx is non-protrusible, and although they lack teeth, one or two styles may be present.
  • Examples of Pharyngobdellida leeches include Erpobdella and Dina.

Leeches have a long history of interaction with humans, being used in various medical and therapeutic practices, including bloodletting and modern reconstructive surgeries.

Class 4- Archiannellida (Gr., arch=first)

  • Archiannellida is a class of annelid worms that exclusively inhabit marine environments. They have an elongated and worm-like body shape. Unlike other annelid groups, Archiannellida typically lack setae and parapodia, which are appendages and bristles found in other classes.
  • The external segmentation in Archiannellida is faintly marked, while the internal segmentation is more distinct due to the presence of coelomic septa. These septa divide the coelom, the body cavity, into compartments.
  • The prostomium, the anterior part of the head, bears two or three tentacles. These tentacles are sensory structures that aid in detecting environmental cues.
  • Archiannellida exhibit sexual reproduction, with the sexes usually separate. However, many species are hermaphroditic, possessing both male and female reproductive organs. The presence of hermaphroditism allows for greater flexibility in mating strategies.
  • In terms of development, Archiannellida typically have a trochophore larval stage. The trochophore larva is a characteristic larval form found in many marine invertebrates.
  • Examples of Archiannellida include Polygordius, Dinophilus, and Protodrilus.
  • Archiannellida play important ecological roles in marine ecosystems, contributing to nutrient cycling and serving as a food source for other organisms. Their presence and abundance contribute to the overall biodiversity and functioning of marine environments.
  • Eg., Dinophilus, Protodrilus

Metamerism in Annelida

  • Metamerism, or the condition of having a segmented body plan, is a characteristic feature of the phylum Annelida. Annelids, such as earthworms and leeches, exhibit a remarkable level of structural repetition throughout their bodies. This metamerism is evident not only in the external appearance but also in the internal organization of these animals.
  • The development of a coelom, a fluid-filled body cavity, is closely associated with the formation of gonadial coelomic sacs positioned on both sides of the gut in Annelida. While in many coelomate animals the coelom is a large perivisceral cavity without clear segmentation, in Annelida, the coelom retains traces of its segmental nature. It is divided into compartments by inter-segmental septa, reflecting the underlying metamerism of the organism.
  • Metamerism in Annelida extends beyond the coelom and encompasses many other systems within the body. Each segment contains homologous structures that repeat in a serial manner. For example, nephridia, which are excretory organs, blood vessels, nerves, reproductive organs, and muscles are all repeated in each segment. This repetitive arrangement allows for a high degree of functional integration among the segments. The segmental structures in Annelida are interconnected and interdependent, working together as a unified functional unit.
  • The body of an annelid is divided into a linear series of segments that are constructed on a similar plan and resemble one another. The segments are stacked one after another, with the youngest segments located towards the posterior end. New segments are continually formed in front of the last segment, which is known as the pygidium. This continuous growth and addition of new segments at the anterior end of the body contribute to the remarkable plasticity and regenerative abilities observed in many annelids.
  • The metamerism in Annelida provides numerous advantages for these organisms. It allows for flexibility and adaptability in movement and locomotion. Each segment can act independently or in coordination with neighboring segments, facilitating precise control over body shape and movement. Metamerism also aids in the distribution of specialized functions across the body, enabling efficient resource allocation and organ specialization.
  • In conclusion, metamerism is a fundamental characteristic of Annelida, with their bodies divided into a series of segments exhibiting repetition of homologous structures. This segmented arrangement extends to the internal organization, including the coelom, reproductive organs, excretory system, and musculature. The interdependence and coordination among these segmental structures contribute to the overall integration and functionality of the organism. Annelids’ ability to continually generate new segments at the anterior end showcases their remarkable regenerative capabilities and adaptive potential.

Coelom in Annelida

  • The coelom, a fluid-filled body cavity, plays a crucial role in the anatomy and physiology of Annelida. It is located between the body wall and the alimentary canal and is formed from segmental vesicles of the mesoderm during embryonic development. The coelom is lined on its outer side by a parietal layer of mesoderm and on its inner side by a visceral layer of mesoderm, together forming the peritoneum.
  • One important function of the coelom in Annelida is its involvement in reproductive processes. The walls of the coelom give rise to reproductive cells, and coelomoducts carry sperm or eggs from the coelom to the exterior. Additionally, excretory organs lead from the coelom to the outside. In certain species of Polychaeta, the coelomic peritoneum gives rise to excretory yellow cells. The coelom itself contains coelomic fluid, which contains amoeboid corpuscles. This fluid serves to absorb nourishment and enables the transport of materials in solution.
  • In Polychaeta and Oligochaeta, the coelom is well-developed and has distinct characteristics. In Polychaeta, the coelom is perivisceral, but it is divided by a series of transverse septa that lie between the body wall and the gut. These septa, which enclose muscle fibers, form double folds of the peritoneum. The coelomic chambers are serially arranged and communicate through spaces around the alimentary canal where the septa are incomplete.
  • In certain species such as Arenicola, only the first three septa and some at the posterior end are present, resulting in an almost uninterrupted coelomic space. In Aphrodite, the coelom is spacious and lined with cilia that facilitate circulation. This development comes at the expense of the blood system.
  • In Oligochaeta, the large perivisceral coelom is divided into compartments by septa that extend between the body wall and the alimentary canal. The first septum in Pheretima lies between segments 4 and 5, allowing the coelom of the first four segments to be continuous. The subsequent eight septa lack apertures, effectively isolating their coelomic chambers from the others. However, from the fourteenth segment onward, the septa have multiple apertures with sphincter muscles, facilitating communication between the coelomic chambers.
  • In Hirudinea, the coelom has undergone modification and no longer exists as a perivisceral cavity. Instead, it is replaced by the formation of botryoidal tissue. However, one primitive leech species (Acanthobdella) retains a perivisceral coelom in the anterior region, complete with septa. In Hirudinaria, the coelom is represented by four longitudinal haemocoelomic channels, their branches, and spaces enclosing the gonads and vasa deferentia.
  • Archiannelida, another class within the phylum Annelida, possess a large coelom divided into chambers by transverse septa.
  • Overall, the coelom in Annelida serves as a vital space for various physiological processes, including reproduction, excretion, and circulation. Its structure and organization differ among different classes and species within the phylum, reflecting the diverse adaptations and evolutionary trajectories of these segmented worms.
Coelom in Annelida
Coelom in Annelida

Segmental Organs of Annelida

Segmental organs play a crucial role in the physiology of Annelida, with nephridia and coelomoducts being the prominent examples.


Nephridia are coiled tubes formed by the invagination of ectoderm, and they reside within the coelom. These structures possess a ciliated lumen that is intracellular. A nephridium typically opens into the coelom through a ciliated funnel or nephrostome, either within the same segment or in the segment just ahead. At the other end, the nephridium opens to the exterior through a nephridiophore. Nephridia primarily function to remove waste from the coelom, although their original purpose may have been water removal from the body.


Coelomoducts, on the other hand, are segmentally repeated mesodermal tubes that have various functions depending on the species. They open into the coelom through a wide ciliated funnel (distinct from a nephrostome) and lead to the exterior at their other end. The lumen of coelomoducts is intercellular. Coelomoducts can serve as excretory organs, combine excretion with the transportation of germ cells to the exterior, or solely function as conduits for germ cells.

Nephridia and Nephromixia:

In certain Polychaeta species, there are nephridia with closed tubes, where their blind ends extend into the coelom. These types of nephridia are known as protonephridia and are observed in organisms such as Phyllodoce and Vanadis. The blind ends of protonephridia are fringed with solenocytes, which are round, ciliated cells connected to the nephridium through thin tubes. These solenocytes possess a long vibratile flagellum and resemble flame cells found in Platyhelminthes.

Nephridia and Nephromixia

However, in many Polychaeta and Oligochaeta species, the nephridia are of the open type, featuring a ciliated nephrostome that opens into the coelom. These are referred to as metanephridia and can be found in organisms like Neanthes and Lumbricus. Some Polychaeta species exhibit compound excretory organs known as nephromixia, which are formed through the fusion of nephridia and coelomoducts. In nephromixia, the functions of excretory organs and genital ducts are combined.

Nephromixia can be classified into three types:

  1. Protonephromixium: The coelomoduct combines with a closed protonephridium, as seen in Aliciopidae and Phyllodoce.
  2. Metanephromixium: The coelomoduct is attached to an open metanephridium, as observed in Hesione.
  3. Mixonephrium or Nephromixium: The coelomoduct and nephridium form a single organ, with the funnel acting as the coelomoduct and the duct functioning as the nephridium. Examples include Capitellidae and Arenicola. The boundary between metanephromixium and mixonephrium is not strictly defined.
Nephridia and Nephromixia

In some Polychaeta, such as Neanthes, a portion of the coelomoduct separates from the metanephromixium and becomes attached to the dorsolateral muscles as a dorsal ciliated organ, aiding in the circulation of coelomic fluid.

In certain tube-dwelling worms like Serpula, there is a division of labor among the nephridia. The anterior segments possess large nephridia responsible for excretion, while the posterior segments house smaller nephridia that exclusively function as genital ducts.

In Oligochaeta and Hirudinea, nephridia and coelomoducts are separate entities. Typically, each segment of these organisms contains a pair of metanephridia, whereas coelomoducts are limited to specific reproductive segments. Nephridia in these organisms may open to the exterior (exonephric nephridia, e.g., Lumbricus) or into the gut (enteronephric nephridia, e.g., Pheretima).

In most earthworms, there is a pair of large-sized metanephridia known as holonephridia or meganephridia in each segment (e.g., Lumbricus). However, in Pheretima, there are numerous small-sized nephridia called meronephridia in each segment. It is believed that the original pair of holonephridia fragmented into multiple meronephridia.

Pheretima exhibits three types of meronephridia:

  1. Anterior segments contain numerous enteronephric meronephridia that open into the pharynx. These meronephridia may have taken on the function of digestive glands and are referred to as peptonephridia.
  2. From the sixth segment onward, there are integumentary exonephric meronephridia in every segment.
  3. In segments after the fourteenth, there are enteronephric meronephridia that open into supra-intestinal excretory ducts that have segmental openings into the intestine.

In Hirudinea, nephridia are typically similar to metanephridia of Oligochaeta, with a ciliated nephrostome opening into a coelomic space (e.g., Hirudo). In Hirudinaria, the nephridia are coiled tubes that open into a bladder, leading to the exterior through a nephridiophore. However, the nephridia in Hirudinaria lack a nephrostome.

Certain Rhynchobdellida species, such as Pantobdella, possess a complex network on the ventral surface of the body. This network gives rise to pairs of branches in each segment, terminating in a ciliated funnel and an opening to the exterior.

Archiannelida, another class within the phylum Annelida, typically have a pair of nephridia in each segment. These nephridia can be closed protonephridia with solenocytes or metanephridia with nephrostomes opening into the coelom (e.g., Polygordius).

Reproduction in Annelida

  • Reproduction in Annelida exhibits a variety of strategies and mechanisms, varying among different classes and species.
  • In Polychaeta, the majority of species have separate sexes. The gonads in Polychaeta are patches of coelomic epithelium and are repeated in multiple segments. During the breeding season, the gonads become conspicuous and proliferate a large number of germ cells. These germ cells detach and fill the coelom, where they undergo maturation in the coelomic fluid. When ripe, the germ cells pass to the exterior either through segmental organs or by rupturing the body wall. Fertilization takes place in seawater, and in many Polychaeta species, a phenomenon known as swarming occurs. Swarming involves the crawling or burrowing worms rising to the surface to discharge their sex cells, after which they sink back to the bottom. This behavior maximizes the chances of fertilizing a large number of eggs. Swarming typically occurs during specific periods, often coinciding with lunar cycles. Following the discharge of gametes, the sexual individuals usually die. Fertilized eggs give rise to trochophore larvae.
  • In Syllidae, the gonads are usually confined to the posterior part of the body, which detaches as a free-swimming zooid. This zooid develops a head but lacks jaws or a pharynx. It lives for a period of time to produce gametes. Some annelids have the ability to regenerate lost body parts, which is often accompanied by a capacity for asexual reproduction. In some forms, asexual reproduction occurs through budding. In the case of Autolytus, a proliferating region at the end of the worm buds off a chain of sexual zooids that detach one by one. Syllis forms many branches through budding, some of which develop a head and sex organs. Notopodia are formed to reconstruct the parapodia. These sexual forms may remain attached to the parent for an extended period or separate from the colony.
  • In Oligochaeta, most species are hermaphrodites, possessing both male and female reproductive organs. The sex cells are discharged into the coelom or into seminal vesicles, which are specialized parts of the coelom. Seminal vesicles are large coelomic sacs that vary in number among different genera. Sometimes, a pair of seminal vesicles may fuse to form a median sperm reservoir, into which the ciliated funnels of vasa deferentia open. Oligochaeta typically have multiple testes and a maximum of two ovaries. Spermathecae are often present to receive spermatozoa from another worm during copulation. The clitellum is a glandular structure developed from the epidermis, which is involved in the formation of cocoons and provides albumen for the nourishment of the embryos. The clitellum may be permanent or develop only during the breeding season. Some Oligochaeta species possess special copulatory setae. Asexual reproduction can also occur in certain Oligochaeta, such as Nais and Chaetogaster, where segments proliferate at the posterior end to form a chain of zooids that eventually separate and acquire sex cells.
  • Hirudinea are also hermaphroditic, with several pairs of testes and two ovaries. The gonads are enclosed in closed coelomic vesicles but are continuous with their ducts. Spermatozoa unite in bundles to form permatophores. Copulation is generally observed in Hirudinea, although hypodermic impregnation may also take place. The clitellum appears during the breeding season, and eggs are laid in cocoons produced by the glands of the clitellum.
  • Archiannelida typically exhibit hermaphroditism, with ovaries occurring in anterior segments and testes located behind them. Therefore, the gonads are restricted to a few segments. In Polygordius, the sexes are separate, with ovaries or testes developing in a few posterior segments, but there are no ducts associated with the reproductive organs.

Difference between Coelom and Pseduocoelom

The main differences between a coelom and a pseudocoelom can be summarized as follows:


  1. It is a fluid-filled perivisceral cavity found in most triploblastic animals, located between the body wall and gut.
  2. The coelom develops from the splitting of the embryonic mesoderm or as an outpocketing of the embryonic archenteron.
  3. The coelomic cavity is lined by coelomic epithelium derived from the embryonic mesoderm.
  4. It has a relationship with the excretory and reproductive organs.
  5. The coelomic fluid contains coelomocytes.

Examples of animals with a coelom include Mollusca, Annelida, Sipuncula, Echiura, Arthropoda, Onychophora, Tardigrada, Echinodermata, Hemichordata, and Chordata.


  1. It is a fluid-filled extracellular body cavity found in some triploblastic animals, occupying the space between the body wall and the intestine.
  2. The pseudocoelom is the remnant of the embryonic blastocoel.
  3. It is lined by endodermal epithelium, not by coelomic epithelium.
  4. It has no direct relationship with the excretory and reproductive organs.
  5. The pseudocoelomic fluid contains pseudocoelomocytes.

Examples of animals with a pseudocoelom include Nematoda, Nematomorpha, Acanthocephala, Rotifera, and Kinorhyncha.

In summary, the key distinction lies in the origin, lining, and functional associations of these fluid-filled cavities within the body structure of different organisms.

LocationPerivisceral cavity between body wall and gutExtracellular cavity between body wall and intestine
DevelopmentDevelops from splitting of embryonic mesoderm or outpocketing of embryonic archenteronRemnant of the embryonic blastocoel
LiningCoelomic epithelium derived from embryonic mesodermEndodermal epithelium
Relationship with Excretory and Reproductive OrgansYesNo
Fluid ContentCoelomic fluid containing coelomocytesPseudocoelomic fluid containing pseudocoelomocytes
ExamplesMollusca, Annelida, Sipuncula, Echiura, Arthropoda, Onychophora, Tardigrada, Echinodermata, Hemichordata, ChordataNematoda, Nematomorpha, Acanthocephala, Rotifera, Kinorhyncha

Examples of Phylum Annelida

Member 1. Polychaetes

Here are some examples of polychaetes, showcasing their diverse characteristics and habitats:

  1. Arenicola:
    • Burrows in sand.
    • Divisible into three regions with distinct features.
    • Builds L or U-shaped tubes within burrows.
    • Liberates reproductive cells through nephridia.
    • Maturity reached after two years with a two-week reproductive phase.
  2. Sabella:
    • Marine, benthic worm known as the peacock worm.
    • Lives within mucin-made tubes encrusted with mud.
    • Possesses gill filaments arising from a semi-circular base.
    • Long-ciliated faecal groove for waste disposal.
    • Reduced prostomium encircled by whorls of tentacles.
  3. Chaetopterus:
    • Marine paddle worm found in Europe and the U.S.A.
    • Lives within a U-shaped, luminiscent tube.
    • Wing and fan-like modified parapodia in the middle region.
    • Reproduction through asexual transverse fission.
    • Can regenerate an entire worm from a single segment.
  4. Amphitrite and Terebella:
    • Both are tube-dwellers but can move like snakes.
    • Prostomium bears retractile multi-grooved tentacles.
    • Multiple pairs of gills, with the third pair small in Terebella.
  5. Serpula and Spirorbis:
    • Live within tubes formed by mucin and calcareous bodies.
    • Possess a crown of tentacles derived from prostomial palps.
    • Feathery tentacles act as gills and brood pouch in Spirorbis.
  6. Aphrodite:
    • Known as “seamouse.”
    • Broad and compact body without hard jaws.
    • Elytra act as plate-like gills and create water currents.
    • Capable of digging burrows up to 50 meters deep.
  7. Eunice:
    • Elongated body with eversible proboscis and tooth plates.
    • Cirrus replaces dorsal parapodium.
    • Notopodium bears a branched gill.
    • Length varies from 3 to 70 cm in different species.
  8. Polynoe:
    • Short and dorsoventrally compressed body.
    • Bioluminescent elytra like Aphrodite.
    • Presence of head with tentacles and peristomial cirri.
    • Proboscis with four jaws.
  9. Tomopteris:
    • Pelagic species with a transparent body.
    • Lateral appendages at the anterior end.
    • Evertible unarmed proboscis.
    • Large paddle-like biramous parapodia without setae.
  10. Autolytus:
    • Small-sized polychaete.
    • Teeth present on the proboscis.
    • Uniramous parapodium.
    • Exhibits alternation of asexual and sexual generations through budding.

These examples demonstrate the wide range of diversity in polychaetes in terms of morphology, habitat, reproductive strategies, and specialized adaptations for their respective lifestyles.

Member 2. Oligochaetes

Here are some examples of oligochaetes, highlighting their characteristics and notable species:

  1. Megascolex:
    • Terrestrial and large in size.
    • Multiple nephridia present in each segment.
  2. Tubifex:
    • Length varies from 2.5-8.5 cm.
    • Red in color.
    • Anterior body remains within a mucus tube, while the posterior end waves rapidly.
    • Withdraws into the tube with slight disturbance.
    • Increased extension from the tube indicates low dissolved oxygen levels.
    • Reproduction is typically asexual.
  3. Allolobophora:
    • Length ranges from 2.5-17 cm.
    • Often ringed with maroon and yellow.
    • Some species are polyploid.
    • A species called A. rosea is parthenogenetic.
  4. Eisenia:
    • Typically 6-13 cm in length.
    • Each segment marked by a red or brown band.
    • Emit a distinctive odor.
  5. Chaetogaster:
    • Usually 0.1-1.5 cm long.
    • Possess distinct segmental grooves and septa.
    • Carnivorous feeding behavior.
    • Internal structures are visible through the transparent integument.
    • Exhibits asexual reproduction at regular intervals, resulting in chains of zooids that later become sexually mature.
  6. Aeolosoma:
    • Prostomium is large and ciliated.
    • Cuticle lacks segmental grooves, and septa are limited to loose muscles.
    • Reddish-yellow droplets are present in the epidermis.
    • Capable of both sexual and asexual reproduction, with asexual reproduction being more common.

While oligochaetes may be less diverse compared to polychaetes, these examples highlight the range of characteristics and adaptations seen within this group of worms.

Member 3. Hirudinea

Here are some examples of hirudineans, commonly known as leeches, along with their characteristics:

  1. Haemadipsa:
    • Variable length, ranging from 2.5 to 8 cm.
    • Terrestrial habitat.
    • Possesses three jaws.
    • Can move very rapidly.
  2. Haemopis:
    • Typically 10-30 cm long.
    • Greenish-black in color.
    • Cylindrical body shape with eyes.
    • Jaws have 14 blunt teeth in two rows.
    • Often parasitic on horses, but some species are predators that can swallow earthworms.
  3. Branchellion:
    • Marine leech that parasitizes ray fishes.
    • Cylindrical body with a bell-like anterior sucker.
    • Body has a narrow and broad region.
    • Foliaceous respiratory plates present on the lateral sides of the body.
  4. Pontobdella:
    • Marine leech that parasitizes bony fish.
    • Cylindrical and club-shaped body.
    • Anterior sucker and truncated posterior sucker.
    • Leathery skin with knob-like warts.
    • Lack eyes and gills.
  5. Acanthobdella:
    • Length typically ranges from 2 to 37 cm.
    • Parasitic on Salmon fish, feeding on blood, skin, and fin tissues.
    • Absence of an anterior sucker, but well-developed posterior sucker.
    • Flattened anterior part with two pairs of setae per segment.
  6. Glossiphonia:
    • Body is dorso-ventrally flattened and oval in outline.
    • Length varies from 0.5 to 3 cm.
    • Parasitic on freshwater snails and aquatic insects.
    • Flat anterior sucker and cup-like posterior sucker.
    • Three pairs of eyes present.
    • Intestine bears four pairs of lateral caeca.
    • Ventral groove carries the eggs and young ones.
  7. Hirudo:
    • Common medicinal leech known as Hirudo medicinalis.
    • Dark green body with six elongated bands of red, yellow, or brown colors.
    • Possesses 100 sharp teeth.
    • Cocoon size is typically 2-3 cm in length.

These examples represent the diversity of leeches, which are hermaphrodites and often adapted for ectoparasitic lifestyles.



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Why do Laboratory incubators need CO2? What is Karyotyping? What are the scope of Microbiology? What is DNA Library? What is Simple Staining? What is Negative Staining? What is Western Blot? What are Transgenic Plants? Breakthrough Discovery: Crystal Cells in Fruit Flies Key to Oxygen Transport What is Northern Blotting?
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