Animal Diversity: Invertebrates
by
Maryam Riasat
PHYLUM ANNELIDA

Phylum Annelida

Characteristics of Phylum Annelida
 Body of annelids have metamerism. Metamerism is a
characteristic that features regularly repeating body
segments.
 This phylum includes earthworms and freshwater
worms (Oligochaeta), leeches (Hirudinea), and
marine worms (Polychaeta)
 Annelids are sometimes called “Bristle worms”,
because of the tiny chitonous bristles that they bear
(except leeches) called setae.

Annelid Anatomy
• The body of an annelid usually
consists of three parts the head
(prostomium and peristomium), a
series of body segments
(metameres) and a terminal
segment called the pygidium.
• Fleshy appendages called
parapodia in some annelids
• Coelomate body cavity
• Bilateral symmetry

Organs and Organ systems
• The annelids show specialization of body structures,
The organ systems are well established.
• Various systems of the body such as the gut, blood
vessels, and nerve cord are constant throughout the
length of the body penetrating each individual
segment.

Locomotion
• Annelids have both longitudinal and circular muscles
which create waves of expansion and contraction
when they move called peristalsis.

Skeletal System
• Fluid in coelom acts as a hydrostatic skeleton

Nervous system
• Consists of a brain, which is connected to a pair of ventral
longitudinal nerve cords, with a ganglion in each segment
(metameric)

Circulatory System
• Annelids are the first group of
invertebrates which have
developed a closed circulatory
system
• Closed circulatory system, in
which the blood is always
enclosed within blood vessels
that run the length of the body
and branch to every segment
• Several hearts (5 in
earthworms) are used to pump
blood through the closed
circuit

Respiratory system
• The breathing system is not present. The
exchange of gases is by diffusion through the
skin into blood capillaries. The skin is kept
moist by mucus and coelomic fluid.

Digestive System
• Unidirectional
• Regional specialization (digestive organs)

Excretory System
• Excretory system consists
of paired (metameric)
metanephridia
• Excretory tubes with
ciliated funnels that
remove waste from the
coelomic fluid; open to
the outside via excretory
pores.

Reproductive System
• Sexes are usually separate with gonads
occurring in each segment
• Some species have gonad specific
segments
• Breeding is usually seasonal (spring or
fall)
• As gametes mature they fill the coelom
and are released by the nephridia
• Fertilization can be internal or external
• Trochophore larvae develop, which are
remarkably similar to the Molluscs
Phylum Annelida

Annelid Ecology
 Annelids are found in the ocean, freshwater, and
in terrestrial soil.
 Often live in burrows in the ground and feeds on
organic matter in soil.
 Others feed on suspended particles that are
trapped on parapodia.
 Some are predaceous, and hunt by hiding in
rocks and corals ambushing their prey.
 Others are bloodsucking carnivores (leeches).

Taxonomic summary
Phylum Annelida have three major classes
• Polychaeta
• Oligochaeta
• Hirudinea

Class Polychaeta
 Name means “many + long hairs”.
 Many setae per body segment.
 Mostly marine and dioecious.
 Most segments with parapodia used for crawling, swimming,
feeding and respiration.
 Examples include the clam worm, scale worms, fireworms,
fanworms (or featherdusters).

Class Oligochaeta
 Name means “few + long hairs”.
 Few setae per body segment.
 Terrestrial and freshwater.
 No head or parapodia.
 Coelom divided by septa.
 Hermaphroditic
 Earthworms

Class Hirudinea
 Name means leeches
 Mostly freshwater, some marine and terrestrial.
 Body usually with posterior and anterior suckers that secrete
anticoagulants and consume blood.
 No parapodia or setae
 Clitellum present
 Hermaphroditic