Phylum Annelida: Definition, Characteristics, Classification and Examples

Phylum Annelida

Phylum Annelida (L., annelus = little ring or F., anneler = to arrange in rings) is a group of bilaterally symmetrical, coelomate, and segmented animals known commonly as segmented worms. Members of this phylum exhibit a body divided into repeated segments called metameres, which give them structural and functional division. Annelids are triploblastic and possess a true coelom, which is divided by septa, and a closed circulatory system. They have a complete digestive system, a well-developed nervous system with a ventral nerve cord, and excretory structures called nephridia. Respiration occurs through the body surface, gills, or parapodia.

Examples of organisms in Phylum Annelida include earthworms (e.g., Lumbricus terrestris), leeches (e.g., Hirudo medicinalis), and marine polychaetes (e.g., Nereis). Annelids are found in terrestrial, freshwater, and marine habitats and play essential roles in ecosystems, such as soil aeration and nutrient cycling.

General Characteristics of Phylum Annelida

• Annelida are mostly aquatic, marine or freshwater, burrowing or living in tubes, some free-living forms.
• Body triploblastic, symmetrical, elongated and vermiform.
• Body is metamerically segmented, externally by transverse grooves and internally by septa into a number of divisions, each division is called segment, metamere or somite.
• Outer covering of the body is cuticle secreted by underlying epidermis.
• Body wall is contractile, consists of an outer epidermis, circular and longitudinal muscles.
• Appendages when present are unjoined.
• Locomotory organs are segmentally arranged, paired setae or chaetae.
• Presence of true coelom divided by intersegmental septa.
• Alimentary canal is tube-like extending straight from mouth to anus.
• Respiration through general body surface, by gills in some forms.
• Blood vascular system is closed type. Blood is red due to the presence of haemoglobin.
• Excretion by paired segmental nephridia which communicate the coelom to the exterior.
• Nervous system consists of a brain and segmental ganglia connected by ventral nerve cord.
• Sexes may be united or separate.

Classification of Phylum Annelida

CLASS 1. POLYCHAETA

1. Polychaeta are marine and carnivorous.
2. Body is elongated and segmented.
3. Head consists of prostomium and peristomium and bears eyes, tentacles, cirri and palps, etc.
4. Setae are numerous and are borne up on lateral prominences of the body wall, known as parapodia.
5. Clitellum is absent.
6. Cirri or branchiae or both may be present for respiration.
7. Coelom is spacious usually divided by intersegmental septa.
8. Alimentary canal is provided with an eversible buccal region and protrusible pharynx.
9. Excretory organs are segmentally paired nephridia.
10. Sexes separate.
11. Fertilization external, free-swimming larval stage trochophore.
12. Asexual reproduction by lateral budding.

Order 1. Errantia

1. Free-swimming, often pelagic, while some living in tubes.
2. All body segments are similar except at anterior and posterior ends.
3. Parapodia, provided with cirri, are equally developed throughout.
4. Head with distinct prostomium which is provided with eyes, tentacles and palps.
5. Pharynx is usually protrusible and armed with chitinous jaws and teeth.
6. Examples: 1. Nereis, 2. Aphrodite, 3. Glycera, 4. Polynoe, 5. Syllis.

Order 2. Sedentaria

1. Burrowing and tube-dwelling forms.
2. Body is divisible into two or more regions with unlike segments and parapodia.
3. Head is small or much modified, without eyes and tentacles, prostomium indistinct.
4. Pharynx is non-protrusible devoid of jaws or teeth.
5. Gills when present, localized to anterior segments.
6. Feeding on plankton or organic detritus.
7. Examples: 1. Chaetopterus, 2. Terebella, 3. Sabella, 4. Arenicola, 5. Serpula, 6. Amphitrite.

CLASS 2. OLIGOCHAETA

1. Mostly terrestrial or some freshwater forms.
2. Body with conspicuous external and internal segmentation.
3. Distinct head, eyes and tentacles are absent.
4. Parapodia are absent.
5. Setae are usually arranged segmentally.
6. Clitellum is usually present.
7. Pharynx is not eversible and without jaws.
8. Hermaphrodite, i.e., sexes united.
9. Development is direct and takes place within cocoons secreted by clitellum.
10. No free larval stage.

Order 1. Archioligochaeta

1. Mostly freshwater forms.
2. Body consists of few segments.
3. Setae are present in bundles.
4. Gizzard is poorly developed, non-muscular or absent.
5. Clitellum is simpler, consists of single layer of cells and situated far forwards.
6. Eye spots are frequently present.
7. Male reproductive openings lie in front of the female reproductive openings.
8. Reproduction asexual and sexual.
9. Examples: 1. Tubifex, 2. Aelosoma.

Order 2. Neooligochaeta

1. Usually terrestrial forms.
2. Body is large and many segmented.
3. Setae are arranged in lumbricine manner.
4. Gizzard well developed.
5. Clitellum is composed of two or more layers of cells and never begins before twelfth segment.
6. Female genital apertures are always on the fourteenth segment and the male pore lies a few segments behind them.
7. Vasa deferentia are elongated extending over the three or four segments.
8. Eye spots are developed.
9. Reproduction sexual. Asexual reproduction is not known.
10. Examples: 1. Pheretima, 2. Eutypheus, 3. Megascolex, 4. Lumbricus.

CLASS 3. HIRUDINEA

1. Mostly ectoparasitic and freshwater forms, while a few are marine feeding upon fishes and other animals.
2. Body is elongated usually flattened dorso-ventrally or cylindrical.
3. Body consists of definite number of segments, each segment breaks up into 2 to 4 rings or annuli.
4. Parapodia and setae are absent.
5. Body is provided with an anterior and a posterior sucker, both situated ventrally.
6. Mouth opens on the ventral surface in the anterior sucker, while anus opens dorsal to the posterior sucker.
7. Hermaphrodite, i.e., sexes united.
8. Reproduction sexual. Asexual reproduction is not known.
9. Eggs are usually laid in cocoons.
10. Development is direct without free-swimming larval stage.

Order 1. Acanthobdellida

1. Mostly parasitic on the fins of salmon fishes.
2. Body comprises thirty segments only.
3. Anterior sucker is absent but posterior sucker is well developed and composed of four segments.
4. Anterior five segments are provided with double rows of setae.
5. Proboscis is short.
6. Body cavity is spacious and incompletely divided by septa.
7. Vascular system consists of dorsal and ventral vessels.
8. Nephridial opening situated on the surface between the segments.
9. Acanthobdellida forms a connecting link between Oligochaeta and Hirudinea.
10. Example: Acantobdella.

Order 2. Rhynchobdellida

1. Parasites on snails, frogs and fishes, marine and freshwater forms.
2. Each typical body segment consists of 3, 6 or 12 rings.
3. Mouth is small median aperture situated in the anterior sucker.
4. Proboscis is protrusible. Jaws are absent.
5. Blood is colourless.
6. Coelom is reduced to sinuses without botryoidal tissues.
7. Examples: 1. Pontobdella, 2. Glossiphonia, 3. Branchellion, 4. Piscicola.

Order 3. Gnathobdellida

1. Freshwater and terrestrial forms.
2. Each typical body segment consists of five rings of annuli.
3. Anterior sucker with three jaws, one median dorsal and two ventrolateral.
4. Proboscis is absent.
5. Blood is red-coloured.
6. Botryoidal tissue present.
7. Examples: 1. Hirudo, 2. Haemopis, 3. Hirudinaria, 4. Herpobdella.

CLASS 4. ARCHIANNELIDA

1. Exclusively marine forms.
2. Body elongated and worm-like.
3. Setae and parapodia are usually absent.
4. External segmentation is slightly marked by faint, while internal segmentation is marked by coelomic septa.
5. Prostomium bears two or three tentacles.
6. Unisexual or hermaphrodite.
7. Larva is typical trochophore.
8. Examples: 1. Polygordius, 2. Protodrilus, 3. Nerilla, 4. Saccocirrus.

CLASS 5. ECHIUROIDEA

1. Marine, found in the mud and sand between rocks in shallow water.
2. The body is more or less cylindrical and is covered by numerous small papillae arranged in rings.
3. Body shows traces of segmentation in adult.
4. Anterior end is provided with a long highly contractile preoral proboscis and a pair of ventral hooked setae.
5. Anus terminal.
6. Parapodia absent.
7. Coelom is large and undivided.
8. Sexes separate.
9. Larva trochophore.
10. Examples: 1. Echiurus, 2. Bonellia.

CLASS 6. SIPUNCULOIDEA

1. Sipunculoidea are found in sand or mud or boring in corals at the sea shore.
2. Body elongated, yellowish or greyish in colour without segmentation.
3. Parapodia and setae are absent.
4. Anterior end introvert provided with mouth encircled by a row of hollow ciliated tentacles.
5. Anus placed on the dorsal surface near the anterior end.
6. Coelom is spacious and undivided.
7. Usually one pair of nephridia.
8. Sexes separate.
9. Larva similar to trochophore.
10. Examples: 1. Sipunculus, 2. Phascolosoma.

CLASS 7. PRIAPULIDA

1. Found widely distributed at moderate depths, burrowing in sand or mud or colder seas.
2. Body cylindrical, worm-like, unsegmented with superficial segmented trunk.
3. Mouth is anterior and anus is posterior.
4. Short introvert without oral tentacles.
5. Posterior end is also provided with a single or double process beset with hollow papillae.
6. Sexes are separate.
7. Development is unknown.
8. Example: Priapulus.

CLASS 8. MYZOSTOMARIA

1. Ecto- or endoparasite on echinoderms specially crinoids.
2. Body is unsegmented, flat and oval or disc-like in shape.
3. Five pairs of parapodia with acicula and hooks present.
4. Four pairs of suckers alternating with parapodia are placed on the ventral surface.
5. Ten pairs of cirri are placed around the edge.
6. Coelom is reduced by parenchymatous tissue.
7. Circulatory and respiratory systems are absent.
8. Single pair of nephridia.
9. Protandric hermaphrodites.
10. Larva trochophore.
11. Example: Myzostoma.