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.
Body is bilaterally symmetrical.
- Body covering:
The body is covered by moist flexible proteinous cuticle layer over epidermis.
The body consists of true coelom (coelom is shozocoel)
- Level organization:
-Organ-system grade of body organization is found,
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 occurs by general body surface and gills.
- Circulatory system:
Blood vascular system is of closed type comprising of hearts and definite blood vessels.
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.
Sexes are separate or united.
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.
- Mostly marine, a few fresh water forms.
- Body has a distinct head with eyes, tentacles and palps.
- Clitellum is absent.
- Alimentary canal with eversiblebuccal region and prostrusible pharynx with or without jaws and teeth.
- Locomotion by paradopia, paddle like fleshy outgrowth from the body.
Example Nereis, Arenicloseetc
- Mostly terrestrial and some fresh water.
- Parapodia are absent instead setae are present.
- Clitellum is usually present.
- Pharynx is non protuisible and is without jaws and teeth.
- They are bisexual.
Example Pheretima posthuma, Lubricus etc.
- They are mostly ecto-parasite and sangivourous but some are fresh water and few are marine.
- Parapodia and setae are absent.
- Body is elongated, usually flattened dorsoventrally or cylindrical.
- Body segmentation with fix number (33).
- They are hermaphrodite.
Example Hirudnaria granulose etc
- These include a group of about one dozen genera of primitive and unknown affinities.
- They are exclusively marine forms.
- Body is elongated and worm like with internal segmentation.
- Prostomium bears two or three tentacles.
- Setae and parapodia are usually absent.