PHYLUM-ANNELIDA
Phylum—Annelida (Annulus — ring; lidos — form) include’
segmented worms. The term ‘Annelida’ was first coined
by Lamarck (1809). It includes about 12000 species of
animals.
General Features
Some important general features of phylum—
Annelida are discussed below
(i) Habit and Habitat They may be aquatic,
terrestrial and free-living or parasitic.
(ii) Body Wall The outermost covering of body is
thin and moist cuticle secreted by the epidermis.
(iii) Metamerism The body is divided into segments
or metameres by ring-like grooves—the annuli. It is
called metameric
segmentation. The
segmentation is external as well as internal.
(iv) Symmetry Annelids are bilaterally symmetrical.
(v) Germ Layers They are triploblastic animals.
(vi) Organisation and Body Plan They show organ
system level of organisation and tube within a
tube body plan.
(vii) Cephalisation These show true cephalisation.
(viii) Body Cavity Annelids have true coelom. It is
formed by splitting of mesodermal epithelium and
such a coelom is called as schizocoelom.
(ix) Skeleton The coelomic fluid in the body cavity
acts as hydrostatic skeleton for the animal.
(x) Locomotion Annelids move by paired, lateral,
hollow, fleshy, appendages in each segment called
parapodia or chitinous setae. Aquatic annelids
like Nereis possess lateral appendages that help in
swimming and have longitudinal and circular
muscle for locomotion.
(xi) Digestive System The digestive tract is straight
and complete, starting from mouth and ending at
anus.
(xii) Respiration It occurs through skin, gills or
parapodia.
(xiii) Circulatory System •They are first animals that
have a closed circulatory system, i.e., the blood
flows through a system of blood vessels.
(xiv) Excretory System The excretory organs are
nephridia in each segment, which helps in excretion
and osmoregulation.
(xv) Nervous System It contains a nerve ring around the
pharynx and a double ventral nerve cord with
ganglion.
(xvi) Reproduction Annelids reproduce sexually. They are
unisexual, e.g., Nereis, and bisexual (hermaphrodite),
e.g., leech, earthworm (Pheretima).