Phylum-Annelida general characters and classification upto classes

• Annelida (L.,annellus = little ring or F., annelar = to arrange in rings)
• Definition : Annelids are triploblastic,bilaterally symmetrical,metamerically segmented,coelomata worms with a thin flexible cuticle around  the body. 
• Number of species : The phylum contains over 8,600 known species.
• Habitat : Annelida are mostly aquatic, marine or freshwater, burrowing or living in tubes, sedentary or free living, some are commensal and parasitic also. 
• GENERAL CHARACTERS :

1. Body triploblastic , bilaterally symmetrical , elongated and vermiform.
2. Body metamerically segmented, externally by transverse grooves  and internally by septa into a number of division ; each division is called a segment, metamere or somite.
3. Outer covering of the body is cuticle secreted by the underlying epidermis.
4. Locomotory organs are segmentally arranged paired setae or chaetae and parapodia in most of the cases.
5. Presence of true schizocoelous coelom.
6. Respiration occurs through body surface in some cases by gills also.
7. Excretion by segmentally arranged nephridia which usually communicate the coelom to exterior.
8. Annelids are usually monoecious,i.e.,hermaphrodite; dioecious or unisexual form also present.
9. Development direct in monoecious forms but indirect in dioecious forms.
10. Asexual reproduction also occurs in some forms.
11. The tactile organs, taste buds, statocyst, photo receptor cells and eyes are the receptor organs.
12. A free swimming trochophore larval stage is characteristic in case of indirect development , while in others this stage is passed during the development.

• Classification upto classes

(Note: Classification to be followed from Rupert and Barnes, 1994,6th edition.)

Phylum Annelida divide into 4  classes.

Class 1: Polychaeta

Class 2: Oligochaeta

Class 3: Hirudinea

Class 4: Archiannelida

          ☆CLASS 1: POLYCHAETA☆

(Gr., poly=many+chaeta=setae/bristles)

1. Body : Body is elongated and segmented.
2. Head : Head consists of prostomium and peristomium and bears eyes, tentacles, cirri and palps, etc.
3. Locomotory organs : Setae are numerous and are borne up on lateral prominences of the body  wall known as parapodia.
4. Clitellum : Clitellum is absent.
5. Respiratory organs: Cirri or branchiae or both may be present for respiration.
6. Coelom : Coelom is spacious usually divided  by  intersegmental septa.
7. Excretory organs : Excretory organs are segmentally paired nephridia.
8.  Sex : sexes separate.
9. Fertilisation : Fertilisation external.
10.  Larval stage : Free swimming larval stage trochophore.
11. Reproduction : Asexual reproduction by lateral budding.

Example : Serpula sp., Neris sp., Sabella sp., Chaetopterus sp., Aphrodite sp.

        ☆CLASS 2 : OLIGOCHAETA☆

(Gr., oligos=few+chaeta=setae/bristles)  1. Habitat :Mostly terrestrial or freshwater forms.

2. Segmentation : Body with conspicuous external and internal segmentation.

3. Distinct head,eyes and tentacles are absent.

4. Locomotory organs : Parapodia are absent. setae are usually arranged segmentally.

5. Clitellum : Clitellum  is usually present.

6.  Sex : Hermaphrodite, i.e., sexes united.

7. Development  : Development is direct and takes place within cocoons secreted by clitellum ; no free larval stage.

Example: Pheretima sp., 

Lumbricus sp., Tubiflex sp., Megascolex sp.

      ☆CLASS 3 : HIRUDINEA☆

               (L., hirudo= a leech)

1. Habit and habitat : Mostly     ectoparasitic and freshwater forms, while few are marine feeding upon fishes and another animals.

2. Shape and size : Body is soft,   elongated, usually flattened dorsoventrally or cylindrical.

3. Segments : Body consists of   definite  number of segments, each segment breaks up into 2 to 4 rings or annuli.

4. Locomotory organs : Parapodia   and   setae are absent.

5. Sucker : Body is provided with  an   anterior and a  posterior sucker, both situated ventrally.

6. Sucker position : Mouth opens on   the ventral surface   in the anterior sucker, while anus opens  dorsal to the posterior sucker.

7. Sex : Hermaphrodite,i.e., sexes   united.

8. Reproduction : Sexual reproduction. Asexual reproduction is not known.

9. Cocoons : Eggs are usually laid in   cocoons.

10. Development : Development is direct without free swimming larval stage.

Example : Hirudo sp., Hirudinaria sp.,

Branchellion sp., Piscicola sp.

 

         ☆Class 4 : Archiannelida ☆

                    (Gr., archi= first)

1. Habitat : Exclusively marine forms.
2. Shape : Body elongated and worm-like.
3. Locomotory organs : Setar and parapodia are usually absent.
4. Segmentation : External segmentation is slightly marked by faint , while internal segmentation is marked by coelomic septa.
5. Head : Prostomium bears 2 or 3 tentacles.
6. Sex : Unisexual or hermaphrodite.

Example : Nerilla sp., Polygordius sp.,

                   Protodrillus sp.