Phylum : Annelida

From Latin words; annulus = ring + eidos = form

Habit and Habitat:
They are mostly aquatic, found in sea as well as in fresh water. Some are terrestrial &ectoparasites.

Body form:
The body is soft, elongated, vermiform sand metomerically symmetrical.

Germ Layers:
They are triploblastic.

Symmetry:
Body is bilaterally symmetrical.

Body covering:
The body is covered by moist flexible proteinous cuticle layer over epidermis.

Coelom:
The body consists of true coelom (coelom is shozocoel)

Level organization:
-Organ-system grade of body organization is found,

Cephalisation:
There is presence of a definite head. It consists of feediry, sensory and nitrous centre.

Digestive system:
Alimentary canal is a straight tube. Digestion is extracellular.

Respiration:
Respiration occurs by general body surface and gills.

Circulatory system:
Blood vascular system is of closed type comprising hearts and definite blood vessels.

Excretion:
Excretion occurs by segmentally repeated wiled tubes, the rephridia.

Nervous system:
The nervous system consists of a pair of dorsal arebralgangia forming a brair, a pair of connectives round the anterior part of alimentary canal (pharynx) and a double segmental ganglionated ventral nerve cord.

Reproduction:
Sexes are separate or united.

Development:
Development may be direct or indirect. If larva is present, as main species, it is trochophore.
Example: Nercis, Hirudinaria etc.

Classification: Phylum is classified into 4 classes on the basis of locomotary organelles.

A. Polychaeta:

1. Mostly marine, a few fresh water forms.
2. Body has a distinct head with eyes, tentacles and palps.
3. Clitellum is absent.
4. Alimentary canal with eversiblebuccal region and prostrusible pharynx with or without jaws and teeth.
5. Locomotion by paradopia, paddle like fleshy outgrowth from the body. Example Nereis, Arenicloseetc

B. Oligochaeta:

1. Mostly terrestrial and some fresh water.
2. Parapodia are absent instead setae are present.
3. Clitellum is usually present.
4. Pharynx is non protuisible and is without jaws and teeth.
5. They are bisexual. Example Pheretima posthuma, Lubricus etc.

C. Hirudinea:

1. They are mostly ecto-parasite and sangivourous but some are fresh water and few are marine.
2. Parapodia and setae are absent.
3. Body is elongated, usually flattened dorsoventrally or cylindrical.
4. Body segmentation with fix number (33).
5. They are hermaphrodite. Example Hirudnaria granulose etc

D. Archiannelida:

1. These include a group of about one dozen genera of primitive and unknown affinities.
2. They are exclusively marine forms.
3. Body is elongated and worm like with internal segmentation.
4. Prostomium bears two or three tentacles.
5. Setae and parapodia are usually absent. Example Protodrilus etc