Phylum Annelida Definition, Characteristics, Classification

Definition of Phylum Annelida 

Annelids are described as a triloblastic, bilaterallysymmetrical, metamerically divided, a coelomate worm with a thin and flexible cuticle that wraps around the body.

  • The annelids, often referred to as ringed or segmented, are an extensive phylum with over 22,000 species in existence, such as earthworms and ragworms and leeches.
  • The species live in and have evolved to different ecologies. Some are in marine environments, such as hydrothermal and tidal vents, while others are found with freshwater, others inhabiting humid terrestrial environments.
  • The Annelids are bilaterally symmetrical triploblastic, coelomate invertebrate organisms. They also possess parapodia for movement.

Characteristics of Phylum Annelida 

  • These are, in general, aquatic, freshwater or marine, some terrestrial and tubicolous. They are burrowing either free-living or sedentary, some parasitic and commensal.
  • It is an elongated body triploblastic, bilaterally symmetricaland truely coelomate and vermiform.
  • The human body has been metamerically divided externally by transverse grooves, and internally by septa into a variety of divisions. Each segment is known as a segment, metamere, or soma.
  • The organ’s organization is a quality system.
  • The epidermis consists of one layer of epithelial cells that are columnar that are covered by a thin cuticle that is not composed of chitin.
  • The body’s wall is contractile or dermo-muscular, consisting of circular outer muscle fibers and the inner longitudinal.
  • Appendages can be joined when they’re present.
  • Organs of the Locomotory family are chitinous bristles that are segmentally repeated that are referred to as setae or Chaetae. They are which are embedded within the skin. They can be irritated by fleshy lateral attachments or even parapodia.
  • The presence of a true coelom that is schizocoelous, usually separated into compartments via transverse septa. It is usually it is well-developed in leeches. Coelomic fluid that contains corpuscles or cells.
  • The alimentary canal has a straight, tube-like and full, stretching from the mouth to the anus. The digestion process is completely extracellular.
  • Respiration is caused by sweaty skin or the gills of head and parapodia.
  • The blood circulation system is closed. The blood color is red due to hemoglobin or erythromycin that is dissolved in plasma.
  • Excretion occurs through metamerically-disposed in coiled tubes, nephridia, which connect to the coelom to the outside.
  • The nervous system is comprised of two cerebral ganglia; the brain, and a the double ventral nerve cord, which has segmentally organized ganglia and the lateral nerves within each segment.
  • Organs that are receptors include sensory organs such as taste buds, statocysts and photoreceptor cells, and occasionally eyes with lenses .
  • The species are monoecious i.e. hermaphroditic, or sexes have separate cleavage spiral and determinate, dioecious or unisexual variants are also available.
  • Their evolution is direct in monoecious forms, but it’s indirect in dioecious forms.
  • Larva, if present, is a trochophore, it is characteristic in the case of indirect development, whereas in other cases this stage is passed through the development.
  • Regeneration is common.
  • In some cases, sexual reproduction is a reality.

Classification of Phylum Annelida

There are more than 8,700 species known to Annelida are classified into four classes principally mostly based on the presence and absence of setae, parapodia metameres and other morphological traits.

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

  • Mostly freshwater, but some marine.
  • Carnivorous
  • Body segmentation is both internal and external.
  • Head is composed of prostomium and peristomium. It has eyes, tentacles, and palps.
  • The Setae are numerous on the parapodia of the lateral.
  • The clitellum does not exist.
  • Branchiae, Cirri, or both are present for breathing.
  • The coelom is usually large and divided by an intersegmental septa.
  • The alimentary canal is characterized by the ever-stable buccal region and an elongated pharynx.
  • The excretory organ has a segmental paired the nephridia.
  • Sexuality is a separate issue. The sexes are separated for a short time and can be seen in numerous segments.
  • Fertilization external.
  • Asexual reproduction through budding lateral.
  • Trochophore larva present.

Polychaeta split into two subclasses: Errantia and sedentaria following Fauvel (1959). But in accordance to Dab (1963) Dab (1963), this distinction is artificial and not an actual one.


Subclass 1. Errantia

  • Crawling, free-swimming, or tube-dwelling, predatory polychaetes.
  • Segmentation is similar, with the exception of the posterior and anterior ends.
  • The prostomium differs from sensory organs.
  • Parapodia, when paired with Cirri, is equally developed all over.
  • Pharynx Protruding, enlarged, and often with teeth and jaws.
  • Examples: Nereis, Aphrodite, Polynoe, Phyllodoce, Tomopteris, Syllis, Eunice, Histriobdella.

Subclass 2. Sedentaraia

  • Burrowing and tube-dwelling forms.
  • Body composed consisting of 2 or more areas, with parapodia and segments that are different.
  • The head is tiny or altered without eyes or tentacles, prostomium is small.
  • There is no acicula or compound setae.
  • Pharynx non-protrusible with no jaws and teeth.
  • Gills, when present can be found to their anterior segment.
  • Food based on plankton or organic debris.
  • Examples: Chaetopterus, Arenicola, Owenia, Sabella, Terebella, Sabellaria, Pomatocerous.

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

  • Mostly freshwater or terrestrial forms.
  • Body with noticeable internal and external segments.
  • Head is indistinct, with no sensory organs.
  • Setae Some, embedded within the skin.
  • Parapodia absent.
  • Glandular clitellum that is present for cocoon development.
  • The pharynx cannot be ever resuscitated and is without jaws.
  • Hermaphroditic i.e. sexes united.
  • Tests that are anterior to Ovaries.
  • The development process is direct. fertilization external (in cocoon); no larval stage.

Order 1. Archioligochaeta

  • The majority of freshwater forms.
  • The body is composed of a couple of segments.
  • Setae can be found in bundles.
  • The gizzard is not well developed and is either non-muscular or completely absent.
  • The clitellum is a simpler structure consisting of just one cell layer and is situated further away from.
  • Eyespots can be seen frequently.
  • Male reproductive openings for males are behind female reproductive open-spots.
  • Sexual and asexual reproduction.
  • Examples: Tubifex, Aelosoma.

Order 2. Neooligochaeta

  • The majority of terrestrial species.
  • The body is huge and is divided into many segments.
  • Setae are handled by a Lumbricine Manner.
  • The gizzard is very well developed.
  • The clitellum consists by two layers or cells. The clitellum never starts before twelfth segments.
  • Female genital apertures are always located on the 14th segment. the male pore is few segments to their left.
  • Vasa differentia are elongated extending over 3 or 4 segments.
  • Eyespots do not develop.
  • Sexual reproduction. Sexual reproduction isn’t well understood.
  • Examples: Pheretima, Eutypheus, Megascolex, Lumbricus.

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

  • Most of them are ectoparasitic, blood-sucking, or carnivore. A few are freshwater, marine or terrestrial.
  • The body is elongated , typically flattened, dorso-ventrally or cylindrical.
  • The body is made up of a set quantity of sections (33). Each segment is broken into 2 to four rings, or annuli.
  • External segmentation without an internal septa.
  • Par setae and podia do not exist.
  • The anterior and posterior sides of the body are adorned with the suckers located in the ventrally.
  • The mouth opens up on the ventral surface of anterior suckers. Anus expands dorsally to sucker posteriors.
  • Coelom is reduced to the filling of botryoidal tissues and the formation of haemocoelomic sinuses.
  • Hermaphrodite, a male and female Gonopore.
  • Fertilization internal.
  • The concept of sexual reproduction isn’t well understood.
  • Eggs are laid every day in cocoons.
  • The development process is straightforward, without the need for a free-swimming larval stage.

Order 1. Acanthobdellida

  • The majority of them are parasites of shark fins.
  • The body is comprised of 30 separate parts.
  • They are primitive, lacking an anterior suckers or proboscis and jaws.
  • Setae with double rows are found in five anterior segments.
  • The cavity of the body is large and is not completely separated by septa.
  • The vascular system comprises ventral and dorsal vessels.
  • Nephridial opening located on the layer between the segments.
  • Examples: A single species and genus (Acanthobdella) parasite on salmon.

Order 2. Rhynchobdellida

  • Parasites of fishes, frogs, snails Freshwater and marine forms.
  • The typical body part consists of 3 (or 12) rings.
  • Mouth is narrow median aperture located inside one of the suckers in front.
  • Protruding proboscis without jaws.
  • Coelom with no compartments.
  • The blood vascular system is separated from the coelomic sinuses.
  • The colorless blood is the only thing that exists.
  • Examples: Placobdella, Helobdella, Piscicola, Branchellion.

Order 3. Gnathobdellia

  • Freshwater and terrestrial forms. Ectoparasitic blood-sucking ectoparasitic lee.
  • A typical body segment is made up of 5 rings , or annuli.
  • Anterior suckers that have 3 jaws, one medial dorsal, as well as 2 ventrolateral.
  • The proboscis is not present.
  • Blood is red in color.
  • Botryoidal tissues present.
  • Examples: Hirudo, Hirudinaria, Haemadipsa, Herpobdella.

Order 4. Pharyngobdellida

  • Terrestrial and aqua. Some predaceous.
  • Pharynx non- protrusible. The teeth are absent, however one or two styles could be present.
  • Examples: Erpobdella, Dina.

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

  • Only marine in form.
  • The body is elongated and has a like a worm.
  • Parapodia and Setae are generally absent.
  • External segmentation is slightly outlined by a faint, whereas inside segmentation can be identified with coelomic septa.
  • Prostomium is a genus that has 2 (or 3 tentacles.
  • Hermaphrodites are usually separated, sexes typically separate.
  • Most often, trochophore larva.
  • Examples: Polygordius, Dinophilus, Protodrilus.

Reproduction and life cycle of Phylum Annelida

Asexual reproduction

  • Polychaetes reproduce asexually by dispersing into several pieces or creating a new organism with the parent remaining an entire organism.
  • Certain oligochaetes like Aulophorus furcatus, appear to reproduce completely asexually while others reproduce sexually in the summer months and sexually in the autumn.
  • The process of reproduction in oligochaetes is always done by splitting into several pieces, and not through budding. Leeches haven’t been observed reproducing sexually. The majority of polychaetes and oligochaetes utilize similar mechanisms to recover from damage.
  • Two genera of polychaete, Chaetopterus and Dodecaceria, can regenerate from just one segment. Other genera are able to regenerate even when they lose their heads.
  • Annelids are the most complicated animal species that regenerate following the most severe injury. However the leeches are not able to regenerate.

Sexual reproduction

  • It is believed that annelids originated as creatures with two separate genders that released sperm and ova in the water via their Nephridia.
  • The fertilized eggs transform into trochophore larvae that remain as plankton.
  • Then they fall to the sea floor and metamorphose into tiny adults. The portion of the trochophore that is between the apical tuft of hair and the prototroch is now it’s prostomium (head) and a tiny region around the trochophore’s anterior transforms into it’s pygidium (tail-piece) and a small strip immediately ahead of that is the growth zone, which produces new segments. The remainder of the trochophore is it’s prototrochium (the segment that houses mouth).
  • About 14% utilize the same external fertilization method, but produce eggs with yolks that will reduce the amount of time the larva is required to spend in the plankton or eggs that allow miniature adults to emerge, rather than larvae.
  • The remaining tend to care for their fertilized eggs up to the time they emerge. Some through the production of jelly-covered masses eggs that they tend in various ways, while others fix eggs to their bodies, and some species keep the eggs inside their bodies until they hatch.
  • The species employ various ways for transfer of sperm, for instance, in certain instances, females accumulate the sperm and release it into the water and in other species, males, there is an ovaries that inject male sperm into female. There is no assurance that this is a true population of polychaetes’ reproduction patterns, but it represents the current understanding of scientists.
  • Certain polychaetes only breed once during their lives, while others breed all the time or over many breeding seasons.
  • While the majority of polychaetes stay of one sex for the entirety of their lives, there is a substantial proportion of species are hermaphrodites who change their sex through their lives.
  • The majority of maturing clitellates (the group of leeches and earthworms) are females who are full hermaphrodites but in a few leech species , young adults are able to function as males, and then become females after a certain age. All species have well-developed gonads and all of them copulate.
  • Earthworms keep their partners the sperm inside the spermathecae (“sperm storage”) and the clitellum forms cocoons that gather ova from the ovaries, and later the spermathecae’s sperm. Fertilization and the development of earthworm eggs occurs inside the cocoon. Leeches eggs are fertilized by the ovaries and later moved to the cocoon. In all clitellates, the cocoon can also produce yolks when eggs are fertilized or produces nutrients during the time they’re developing. All Clitellates develop as mini adults instead of larvae.

Body wall, chaetae and parapodia of Phylum Annelida

  • The cuticles of Annelids are composed of collagen fibers, typically in layers which spiral in alternating directions in order that the fibers cross each one. They are secreted by the deep epidermis of one cell (outermost the skin).
  • Some marine annelids which reside in tubes do not have cuticles. However, their tubes share the same structure and mucus-secreting glands located in the epidermis guard their skins.
  • The epidermis, or the skin layer, is the dermis. It is comprised of connective tissue that is, a mix of cells as well as non-cellular components like collagen.
  • Below there are 2 layers of muscle that originate from the lining of body’s coelom (body cavity) circular muscles create a segment that is longer and slimmer when they contract and beneath them are muscles that are longitudinal, typically four distinct strips that contract to reduce the length of the segment and make it larger.
  • A few annelids also have the internal muscles with oblique connections that connect the bottom and the upper part of your body to the sides.
  • Setae (“hairs”) from Annelids extend out of the epidermis to give traction and other abilities. The most basic are not joined and form paired bundles at the upper and lower part of the segment.
  • Parapodia (“limbs”) that make up an annelid that are equipped with them usually carry more complex chetae on their tips, for instance, they can be joined hooks, comb-like, or joined. Chetae are comprised of moderately flexible b-chitin , and are created by follicles each with an Chetoblast (“hair-forming”) hair-forming cell located at the bottom. It also has muscles that expand as well as extend the cheta.
  • Chetoblasts create chetae through making microvilli, which are hair extensions that expand the surface area that is available for secreting the cheta.
  • After the cheta is completed The microvilli disappear into the Chetoblast, leaving parallel tunnels that cover the entire length of the cheta.
  • Therefore, annelids’ chetae can be distinctively different from setae (“bristles”) of arthropods. These are composed out of more rigid a-chitin. with a single internal space, and placed on joints that are flexible and have small pits within the cuticle.
  • The majority of polychaetes possess parapodia, which function as limbs. However, other major annelid groups do not have them.
  • Parapodia are extensions that are not joined of the body’s wall and their muscles derive by the circular muscle in the body.
  • They are usually held in place with one or two massive thick chetae. The parapodia that are found in burrowing and tube-dwelling polychaetes can be described as ridges with tips that bear hooked chetae.
  • For swimmers and crawlers who are active, the parapodia are usually divided into two large upper and lower paddles with a very small trunk. The paddles are usually surrounded by chetae and , occasionally, by Cirri (fused clusters that are cilia) and Gills.
Body wall, chaetae and parapodia
Body wall, chaetae and parapodia | Image Source:

Segmentation of Phylum Annelida

  • The majority of the body of an annelid comprises of segments that are nearly identical, with the same set of internal organs as well as external Chaetae (Greek”khaite” which means “hair”) and, in certain species the appendages.
  • The rearmost and frontmost segments aren’t considered to be genuine segments because they don’t contain the organs that are typical and don’t develop similarly to the genuine segments.
  • The most frontal part, referred to as the prostomium (Greek pro- which means “in front of” and the word stoma referring to “mouth”) contains the organs of the brain and senses and the backmost known as the pygidium (Greek pugidion which means “little tail”) or periproct, houses the anus typically on the side that is lower.
  • The section that is located behind the prostomium is known as the peristomium (Greek peri- which means “around” and stoma meaning “mouth”), is thought by some scientists to be not a genuine segment however in some polychaetes, the peristomium is characterized by chetae and appendages similar to those of other segments.
  • The segments grow each one at a time, starting in a zone of growth in front of the pygidium such that an annelid’s newest segment is located just ahead of the zone that is growing, while the peristomium is the most old. This is known as Teloblastic Growth.
  • Certain groups of annelids including all leeches have fixed numbers of segments. Other groups have segments that they can add throughout their lives.
  • The name phylum comes by its Latin word annelus which translates to “little ring”.

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