Annelida

Annelida is composed of two words i.e. annulus which means ring and eidos which means form. Lamarck coined the term Annelida.

General characteristic features of phylum Annelida:

• They are elongated and metamerically segmented.
• They are mostly aquatic and some are terrestrial and burrowing. Few are ectoparasites sucking the blood from vertebrates.
• They are triploblastic.
• They are bilaterally symmetrical.
• It is true coelom type formed by mesoderm. Such coelom is called schizocoelous.
• It takes place by setae or parapodia or suckers.
• Nutrition is a holozoic type.
• The alimentary canal is straight and well developed. Digestion is entirely extracellular.
• Respiration takes place through a moist body surface by diffusion.
• Blood vascular system is closed type consisting of hearts and blood vessels.
• Excretion takes place through segmentally arranged nephridia.
• The body contains metameric segmentation.
• Presence of distinct head. The nervous system consists of a pair of cerebral ganglia and double solid ventral nerve cord.
• Reproduction is usually hermaphrodite. Some cases, there is unisexual. Fertilization is cross and internal.
• Development may be direct or indirect with the larval stage as trochophore.

Classification:

Annelida is classified into four classes on the basis of locomotory organs.

Polychaeta:

• Locomotory organs are parapodia.
• Mainly marine, some freshwater.
• Clitellum is absent.
• Sexes are separate.
• Fertilization is external. Development is indirect.
• Example: Nereis, Amphitrite

Oligochaeta:

• Locomotory organs are setae found in each segment.
• Mostly terrestrial, some fresh water.
• Clitellum is present.
• They are hermaphrodite.
• Fertilization is external, occurs in a cocoon.
• Development is direct.
• Example: Pheretima, Negascolen.

Hirudinea:

• Locomotory organs are suckers.
• Mostly freshwater and terrestrial, some marine.
• They are mostly ecoparasites having suckers.
• Clitellum is only seen during a breeding season.
• Fertilization is external, occurs in a cocoon.
• Example: Hirudinaria, Hirudo.

Arachiannelid:

• Segmentation is internal only.
• Exclusively marine.
• Parapodia and setae are present.
• Tentacles are present.
• Sexes are separate. Development is indirect.
• Example: Polygordius, Protgdrillus.