Classification of Annelida

Classification of Annelida:

Class 1. Polychaeta

(Gr., poly = many + chaete = bristles):

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 Para podia.

5. Clitellum is absent.

6. Cirri or branchiae or both may be present for respiration.

7. Coelom is spacious usually divided by inter-segmental septa.

8. Alimentary canal is provided with an eversible buccal region and protrusible pharynx.

9. Excretory organs are segmentally paired nephridia.

10. Sexes separate, fertilisation external, free swimming larval stage trochophore.

11. 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.

Examples:

Nereis, Aphrodite, Glycera, Polynoe, Syllis.

Order 2. Sedentaria:

1. Burrowing and tube dwelling forms.

2. Body is divisible into two or more regions with unlike segments and Para podia.

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, localised to the anterior segments.

6. Feeding on plankton or organic detritus.

Examples:

Chaetopterus, Terebella, Sabella, Arenicola, Serpula, Amphitrite, Spirorbis.

Class 2. Oligochaeta:

(Gr., oligos =few + chaete = setae):

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; 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.

Examples:

Tubifex, Aelosoma.

Order 2. Neooligochaeta:

1. Usually terrestrial forms.

2. Body is large and many segmented.

3. Setae are arranged in lumbricine manner.

4. Gizzard is 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 never developed.

9. Reproduction sexual. Asexual reprodu­ction is not known

Examples:

Pheretima, Eutypheus, Megascolex, Lumbricus.

Class 3. Hirudinea:

(L., hirudo = a leech):

1. Mostly ectoparasitic and freshwater forms, while few are marine feeding upon fishes and other animals.

2. Body is elongated usually flattened dorsoventrally 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; repro­duction sexual.

8. Asexual reproduction is not known.

9. Eggs are usually laid in cocoons.

10. Development is direct without free swimming larval stage.

Order 1. Acanthobdeilida:

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 vessel.

8. Nephridial opening situated on the surface between the segments.

9. Acanthobdeilida forms a connecting link between Oligochaeta and Hirudinea.

Example:

Acanthobdeilida.

Order 2. Rhvnchohdellida:

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 prostrusible; jaws are absent.

5. Blood is colourless.

6. Goelom is reduced to sinuses without botryoidal tissues.

Examples:

Pontobdella, Glossiphonia, Branchellion, Piscicola.

Order 3. Gnathobdetlida:

1. Freshwater and terrestrial forms.

2. Each typical body segment consists of five rings or annuli.

3. Anterior sucker with three jaws, one median dorsal and two ventro-lateral.

4. Proboscis is absent.

5. Blood is red coloured.

6. Botryoidal tissue present.

Examples:

Hirudo, Haemopis,

Hirudinaria, Herpobdella.

Class 4. Archiannelida

(Gr., archi = first):

1. Exclusively marine forms.

2. Body elongated and worm-like.

3. Setae and Para podia 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.

Examples:

Polygordius, Protodrillus, Nerilla, Saccocirrus