Definition and Introduction
Bilateral and protosomial eucoelomate eumetazoans whose long narrow and wormlike body is division into ring like true or metameric segments (Gr. Annelus= little ring) and the skin usually bears enjointed chitinous appendages , termed setae.
Commonly known as segmented worms. About 9,000 species known.
Linnaeus (1758) included all soft bodied worms in Vermes Lamarck (1801) established phylum Annelida for higher types or worms.
1. Annelids occur widely in water or moist soil. Some free living some burrowing and a few parasitic.
2. Body usually long , slender and distinctly segmented. Body cavity ( coelom and many internal organs also exhibit metameric segmentation .
3. Soft and extensile body triploblastic and bilateral.
4. Muscular and contractile body wall covered by a thin moist cuticle containing secreted unjointed chitinous setae. A distinct columnar epidermis and both longitudinal and circular muscle layers present.
5. Body cavity is true coelom lined by sesodermal epithelium ( peritoneum) phylogentically, annelids are, perhaps, the first animals to acquire a ture schizocoelic coelom, In most annelids ,it is divided into segmental chambers by inter segmental septa. Coelomic fluid contains cells ( corpuscles ) .
6. Alimentary canal straight with anterior mouth and posterior anus .Due to spacious, fluid filled body cavity between body wall and alimentary canal, the body grossly appears like a tube within a tube in section.
7. A distinct and closed type of blood vascular system present. Blood of red colour with haemoglobin dissolved in plasma. Free amoeboid blood corpuscles present, but no red blood corpuscles.
8. Respiration through moist skin some possess gills.
9. Excretion by minute and coiled paired tubules of ectodermal origin ,called metanephridia.
10. Nervous system uncludes nerve ring near anterior end. Two ventral nerve cords extending throughout body length from nerve ring, and segmental ganglionic swellings upon nerve cords, representing true metamerism. Several types of sensory cells and organs found.
11. Sexes separate or united .Reproductive organs develop from coelomic epithelium.
12. Development mostly direct. When present , the larval stage is trochophore.