NEET Biology Animal Kingdom Notes - Phylum-Annelida

Phylum-Annelida

Phylum Annelida.

(i) Introduction : Bilateral and protostomial eucoelomate eumetazoans whose long, narrow and worm like body is divided into ring-like, true or metameric segments (Gr., annelus = little ring), and the skin usually bears unjointed, chitinous appendages, termed setae. Commonly known as ?segmented worms?. About 9,000 species known.

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

         (iii) Salient features  

         (1) Annelids are bilaterally symmetrical animals

         (2) They have organ-system grade of organization

         (3) They are coelomate 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. 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

         (10) Blood vascular system is a closed type

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

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

         (13) Mostly annelids are hermaphrodites

(14) The gonoducts are formed from coelom (coelomoducts). The coelomoducts have connection with nephridis.

         (15) Regeneration is common character in this phylum

         (16) Their development is direct or indirect.

       (iv) Classification of annelida

       (a) Class 1. Polychaeta

         (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) Clitellum is absent

         (6) Cirri or branchiae or both may be present for respiration

         (7) Coelom is spacious usually divided by inter segmented septa

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

         (9) Excretory organs are segmentally paired nephridia

         (10) Saxes are separate

         (11) Fertilization is external; free swimming larval stage is trochophore

         (12) Asexual reproduction occurs by budding.

         Examples : Nereis, Aphrodite, Polynae, Chaetopterus

q Trochophore larva present in polychaeta (Neries). Aphrodite is commonly known as Sea mouse.

       (b) Class 2. Oligochaeta

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

(2) Body has conspicuous external and internal segmentation.

         (3) Dsitinct 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) They are hermaphrodites.

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

         (10) No free larval stage

         Examples : Tubifer, Pheretima, (All earthworms).

       (c) Class 3. Hirudinea

(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) 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

         Examples : Acantaobdella, Glossiphonia (All leeches)

       (d) Class 4 Archiannelida

         (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 : Polygoridus, Protodrillus, Nerilla, Saccocirrus, etc.