Phylum Annelida- Characteristics, Classification, Examples

Phylum Annelida definition

Phylum Annelida are described as coelomate worms with a thin, flexible cuticle all around that are triploblastic, bilaterally symmetrical, and segmented metamerically.

Phylum Annelida Characteristics

  • Most of them are aquatic; some are freshwater or marine; some are terrestrial; some are sedentary or free-living; and some are commensal and parasitic.
  • The body is triloblastic, vermiform, really coelomate, elongated, bilaterally symmetric, and vermiform.
  • The body is divided into several divisions by septa and transverse grooves on the outside, each of which is referred to as a segment, metamere, or somite.
  • The body is organised according to organ grade.
  • The epidermis is composed of a single layer of columnar epithelial cells and is protected by a thin, non-chitinous cuticle.
  • The dermo-muscular or contractile body wall is made up of circular and longitudinal outer muscle fibres.
  • If present, appendages are joined.
  • Setae or chaetae, which are imbedded in the epidermis, are chitinous bristles that are segmentally repeated. Lateral fleshy appendages or parapodia may bear it.
  • True schizocoelous coelom is present and is frequently separated into compartments by transverse septa. Leeches have evolved pretty nicely.
  • Cells or corpuscles in the coelomic fluid.
  • The alimentary canal is a continuous, straight tube that runs from the mouth to the anus. Digestion occurs only extracellularly.
  • Through wet skin or the gills of the parapodia and head, one breathes.
  • The blood vessels are part of a closed system. Plasma-dissolved erythromycin or haemoglobin are the causes of the red colour of blood.
  • Nephridia, which transmit the coelom to the outside, are metamerically distributed coiled tubes that are used for excretion.
  • The neurological system consists of two cerebral ganglia, a segmentally organised brain, and a double ventral nerve cord with lateral nerves in each segment.
  • Tactile organs, taste buds, statocysts, photoreceptor cells, and occasionally eyes with lenses are examples of receptor organs.
  • They are hermaphrodite or monoecious, with spiral cleavage and definite sex divisions; a dioecious or unisexual form is also found.
  • In monoecious forms, they develop directly, but in dioecious forms, they evolve indirectly.
  • When a trochophore is present, the larva exhibits signs of indirect development, although in other cases, this stage has already been reached.
  • Renewal frequently occurs.
  • Some people can reproduce in an asexual way.

Classification of the Phylum Annelida

The roughly 8,700 species of Annelida that are now recognised are grouped into four major groups, mostly based on the presence or lack of parapodia, setae, metameres, and other physical characteristics.

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

  • mostly in the sea, with some fresh water.
  • Carnivorous
  • There are internal and exterior body segments.
  • The head has eyes, cirri, and palps and is made up of prostomium and peristomium.
  • Numerous setae cover the lateral parapodia.
  • Clitellum is not present.
  • For respiration, cirri, branchiae, or both, may be present.
  • Intersegmental septa often separate the coelom, which is a large space.
  • The pharynx and eversible buccal area were features of the alimentary canal.
  • Nephridia are segmentally paired organs that excrete waste.
  • Sexes are distinct. Gonads are transient and come in various sizes.
  • fertilisation from outside.
  • Through lateral budding, an asexual reproduction.
  • Present Trochophore larva.
  • Following Fauvel, Polychaeta was separated into the subclasses Errantia and Sedentaria (1959). However, he claims that this divide is artificial and not natural.

Subclass 1. Errantia

  • predatory polychaetes that are free-swimming, crawling, burrowing, or tube-dwelling.
  • Similar segmentation, with the exception of the front and back ends.
  • With sensory organs, the prostomium is unique.
  • Cirri-provided parapodia are evenly developed all throughout.
  • Protruding, enormous, and typically equipped with teeth and jaws,
  • Nereis, Aphrodite, Polynoe, Phyllodoce, Tomopteris, Syllis, Eunice, and Histriobdella are a few examples.

Subclass 2. Sedentaraia

  • Tube-dwelling and burrowing are forms.
  • The body composed of two or more sections, each containing parapodia and dissimilar segments.
  • Small or greatly altered head without eyes and tentacles. Small prostomium.
  • Complex setae and no acicula.
  • Without jaws and teeth, the pharynx cannot protrude.
  • When present, gills are restricted to the front segments.
  • feeding on organic waste or plankton.
  • Chaetopterus, Arenicola, Owenia, Sabella, Terebella, Sabellaria, and Pomatocerous are a few examples. 

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

  • Most of the types are terrestrial or freshwater.
  • Internal and exterior body segments are clearly segmented.
  • Head lacking sensory organs and blurry.
  • Few, imbedded in the skin, setae.
  • Absence of parapodia
  • There is a glandular clitellum for cocoon production.
  • The pharynx lacks jaws and is not eversible.
  • Hermaphrodite, or possessing both sexes.
  • The testes are anterior to the ovaries.
  • Direct development is used. No larval stage; external (in the cocoon) fertilisation.

Order 1. Archioligochaeta

  • Freshwater forms predominate.
  • The body is divided into a few sections.
  • There are bundles of setae.
  • The gizzard is missing, has no muscles, or is not well-developed.
  • The clitellum is less complicated, just has one layer of cells, and is located far away.
  • There are common eyespots.
  • In front of the female reproductive apertures are the male reproductive openings.
  • Sexual and asexual reproduction
  • Examples include Tubifex and Aelosoma

Order 2. Neooligochaeta

  • typically earthly in nature.
  • The body has several segments and is rather massive.
  • The management of setae is lumbricine.
  • The gizzard has grown wonderfully.
  • The clitellum never develops before the twelfth segment and is made up of two or more layers of cells.
  • The male pore is always located a few segments after the female genital opening, which is always on segment 14.
  • Vasa differentia has a lengthy shape that spans three or four segments.
  • Eyespot development never occurs.
  • Sexual reproduction. It is unknown how asexual reproduction works.
  • Pheretima, Eutypheus, Megascolex, and Lumbricus are among the examples.

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

  • primarily blood-sucking, carnivorous, or ectoparasitic. Few are terrestrial, freshwater, or marine.
  • The body is cylindrical or dorso-ventrally flattened and elongated.
  • There are a certain number of segments that make up the body (33). Each segment is divided into two to four annuli, or rings.
  • segmentation without internal septa on the outside.
  • Setae and par podia are not present.
  • Suckers are located ventrally on the body’s anterior and posterior ends.
  • Anus opens dorsally to the posterior suckers, whereas the mouth opens on the ventral side of the anterior suckers.
  • Because of the filling of the coelom by botryoidal tissues, hemocoelomic sinuses are formed.
  • One male and one female gonopore on a hermaphrodite.
  • intrinsic feminization.
  • It is unknown how asexual reproduction works.
  • In cocoons, eggs are always deposited.
  • Without a stage of free-swimming larvae, development is direct.

Order 1. Acanthobdellida

  • mostly parasitic on salmon fish fins.
  • There are just 30 segments in the body.
  • They lack jaws, anterior suckers, and a proboscis, making them primitive.
  • Five anterior segments have double rows of setae.
  • The body cavity is roomy and only loosely segmented by septa.
  • The dorsal and ventral vessels make up the vascular system.
  • Nephridial opening is located between the segments on the surface.
  • Examples include the salmon parasite Acanthobdella, a single genus and species.

Order 2. Rhynchobdellida

  • marine and freshwater parasites that live on snails, frogs, and fish.
  • Typically, each body section has 3, 6, or 12 rings.
  • The anterior suckers include a tiny median opening that serves as the mouth.
  • It has a proboscis that sticks out and has no jaws.
  • Without compartments, Coelom
  • Coelomic sinuses are separated from the blood vessels.
  • Blood has no colour.
  • Placobdella, Helobdella, Piscicola, and Branchellion are a few examples.

Order 3. Gnathobdellia

  • terrestrial and freshwater forms. Blood-sucking ectoparasitic leeches.
  • An average body segment has five rings, called annuli.
  • Anterior suckers, have three jaws: two ventrolateral and one middle dorsal.
  • There is no proboscis.
  • Red is the colour of blood.
  • Current Botryoidal tissues
  • A few examples include Hirudo, Hirudinaria, Haemadipsa, and Herpobdella.

Order 4: Pharyngobdellida

  • both land and water. A few predatory
  • Pharynx does not protrude. There may be one or two styles, but no teeth.
  • Consider Erpobdella and Dina.

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

  • only in marine form.
  • Long and worm-like in shape.
  • Parapodia and setae are typically lacking.
  • Faint and coelomic septa serve as markers for internal and external segmentation, respectively.
  • The protomium has two or three tentacles.
  • Hermaphrodites generally have distinct sexes.
  • Trochophore larvae are typical.
  • Examples are Protodrilus, Polygordius, and Dinophilus.


  • 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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