PHYLUM ANNELIDA

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.

Brief History

Linnaeus (1758) included all  soft  bodied worms  in Vermes  Lamarck (1801)  established phylum  Annelida for higher types   or worms.

Important Characters

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.