11th Class Biology Animal Kingdom Phylum Annelida - The Segmented Animals

 (L.annelus = ring, eidos = form)

Brief History : Linnaeus (1758) included all soft–bodied worms in “Vermes”. Lamarck (1801) established phylum annelida for higher types of worms.

General characters

(1) Annelids are bilaterally symmetrical animals.

(2) They have organ-system grade of organization.

(3) They are coelomate (schizocoelomate) animals.

(4) They have triploblastic body wall.

(5) The muscle layers are thick in the body wall. Hence the body wall is said to be dermomuscular.

(6) The body is divided into a numerous segments called the metameres or somites. The segmentation is known as metamerism.

(7) The body is covered with a thin cuticle.

(8) Locomotory organs are setae.

(9) Digestive system is well developed. These have tube-within-a-tube body plan.

(10) Blood vascular system is a closed type

(11) Excretory system is formed of segmentally arranged nephridia.

(12) These always show cutaneous or skin respiration.

(13) Nervous system is formed of a pair of cerebral ganglia (brain) and a double ventral nerve cord.

(14) Mostly annelids are hermaphrodites. Fertilization is generally cross and may be external or internal.

(15) The gonoducts are formed from coelom (coelomoducts). The coelomoducts have connection with nephridia.

(16) Regeneration is common character in this phylum.

(17) Their development is direct or indirect and includes a free-swimming trochophore larva.

Classification of annelida : On the basis of position and arrangement of setae when present, absence and presence of sense organ, phylum annelida has been divided into four classes –

Class 1. Polychaeta (Gr. polus, many, chaite, hair)

(1) Polychaeta are marine and carnivorous.

(2) Body is elongated and segmented.

(3) Head consists of prostomium and peristomium and bear eyes, tentacles, cirri and palps, etc.

(4) Setae are numerous and are borne up on lateral prominances of the body wall known as parapodia.

(5) Locomotory organs are parapodia.

(6) Clitellum is absent.

(7) Cirri or branchiae or both may be present for respiration.

(8) Coelom is spacious usually divided by inter segmented septa.

(9) Alimentary canal is provided with an eversible buccal region and protrusible pharynx.

(10) Excretory organs are segmentally paired nephridia.

(11) Sexes are separate.

(12) Fertilization is external; free swimming larval stage is trochophore.

(13) Asexual reproduction occurs by budding.

Examples : Nereis, Aphrodite, Polynae, Chaetopterus, Glycera, Arenicola, Amphitrite, Terebella, Sabella, Eunice, etc.

• Arenicola, Amphitrite and Terebella have external gills.
• Chaetopterus exhibits luminescence and great power of regeneration. 

Class 2. Oligochaeta (Gr. oligi, few)

(1) They are mostly terrestrial or some fresh water forms.

(2) Body has conspicuous external and internal segmentation.

(3) Distinct head, eyes and tentacles are absent.

(4) Parapodia are absent.

(5) Locomotory organs are setae.

(6) Setae are usually arranged segmentally.

(7) Clitellum is usually present.

(8) Pharynx is not eversible and without jaws.

(9) They are hermaphrodites.

(10) Development is direct and takes place within cocoons secreted by clitellum.

(11) No free larval stage

Examples : Tubifex, Dero, Pheretima, (Indian earthworms), Lumbricus (European earthworm).

• Tubifex and Dero are fresh water forms. Tubifex can live in polluted water where oxygen availability is poor because its has a large amount of haemoglobin in blood.

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

(1) This class includes mostly ectoparasitic and fresh water forms, while few are marine, feeding upon fishes and other animals.

(2) Body is elongated usually flattened dorso–ventrally or cylindrical.

(3) Body consists or definite number of segments, each segments 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) Coelom is reduced by botryoidal tissue.

(7) Mouth opens on the ventral surface in the anterior sucker, while anus opens dorsal to the posterior sucker.

(8) Locomotory organs are suckers.

(9) Hermaphrodite i.e., sexes united.

(10) Reproduction sexual. Asexual reproduction is unknown.

(11) Eggs are usually laid in cocoons.

(12) Development is direct without free swimming larval stage.

Examples : Acantobdella, Glossiphonia (Fresh water leeches), pontobdella, Haemodipsa, etc.

• Haemodipsa is terrestrial leech.
• Pontobdella is a ectoparasite on elasmobranchi fishes.
• Acanthobdella is a ectoparasite of salmon fish.

Class 4. Archiannelida (Gr. archi, primitive)

(1) They are 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 trochopore.

Examples : Polygordius, Protodrillus, Nerilla, Saccocirrus, etc.

• Polygordius is a primitive Archiannelid or living fossil.