ZLY 103
Animal Diversity
Phylum Annelida
Introduction
Phylum Annelida (an-nel′i-da) (L. annelus, little ring,
+ ida, pl. suffix) consists of the segmented worms.
Annelids are diverse numbering about 15,000
species.
The most familiar are earthworms and freshwater
worms (class Oligochaeta) and leeches (class
Hirudinea).
Approximately 2/3rds of Annelids are marine worms
(class Polychaeta), (less familiar).
Among the latter are many curious members; some
are strange, even grotesque, whereas others are
graceful and beautiful
Introduction
They include clamworms, plumed worms,
parchment worms, scaleworms, lugworms, and
many others.
Annelids are true protostome coelomates belonging
to super phylum Lophotrochozoa, which exhibit
spiral cleavage and mosaic development.
Highly developed group with a centralized nervous
system and a complex circulatory.
Annelids have a spiral cleavage and determinate
development.
First group to exhibit true metameric segmentation
Importance & Biological Contributions
 Annelids opened a wide complexity and variability in
forms and structure, viz:-
Metamerism (Segmentation) foundation for specialization
Presence of Coelom (fluid filled cavity) between 2 layers:
splanchnic & somatic mesoderm respectively
 Constant body segmentation, structurally and
morphologically similar
 Each metamere components work in unison,
segments are interdependent
 Youngest segment appear at the posterior end and
new segments originates from the pygidium
Evolutionary importance of Metamerism
1. Advent of greater complexity in form and
structure.
2. Specialization in segmentation (Tagmatization).
3. Metamerism brought built-in-redundancy
4. Metamerism opens the door for wide advances in
functions as seen in Arthropoda.
5. The evolution of true coelom began with Annelids
but came with some consequences:
– Total separation of the gut and associated muscles from
body wall
– Complexity in organization (ends dorsoventral body)
– Diffusion no longer solves gaseous exchange and feeding
Advantages of Coelomic Inclusion
1. Reproduction: Gonads, gametes shed and mature
within the coelomic cavity.
2. Acccomodation: Various organs perform biological
functions independently
3. Excretion & Osmoregulation: Coelom serves as
temporary storage sites of metabolic wastes.
4. Support, Hydrostatic skeleton & Protection: Coelom
offers shape, support and protection to delicate
organs.
5. Movement: Body wall movement without effect on
the gut.
Characteristics
 They are triploblastic, bilateral, metameric segmented
coelomates.
• They are externally marked by circular rings called
annuli (the name of the phylum refers to this
characteristic).
• Annelids are sometimes called “bristle worms” because,
(except leeches), most annelids bear tiny chitinous
bristles called setae.
• Short needlelike setae help anchor segments during
locomotion and long hair like setae aid aquatic forms
in swimming.
• The coelom develops embryonically as a split in the
mesoderm on each side of the gut (schizocoel).
Characteristics
• They burrow or live in secreted tubes, stiff setae also aid
in preventing the worm from being pulled out of soil.
• Habitats
– Mostly aquatic: marine or freshwater,
– Some terrestrial: Burrow or live in tubes, Sedentary, free-living
– Few are parasitic
• Organ-system grade of body organization.
• Excretory system is the nephridium: Arranged paired
on each body segment.
• Central Nervous System (CNS) comprised of solid,
ventral, double ganglionated nerve cord and paired
dorsal cerebral ganglia (Brain).
Characteristics
• Alimentary system is tube-like, complete, extending
from mouth to anus (Open gut system).
• Blood system is closed circulatory
• Some monoecious: hermaphrodites – Direct development
• Dioecious: Unisex – Indirect development
• Free-swimming Trocophore larva stage in indirect
development (next slide).
• Some exhibit asexual reproduction by budding
Polychaete Trochophore Larvae
Phylum Annelida
General Body Plan
• ectoderm
• mesoderm
• endoderm
• coelom
• septum
one metamere
l.s. segmented worm
Body Cavities
Eucoelomates
Body cavity completely
lined with mesoderm
Mesoderm
coelom
gut
Ectoderm
Endoderm
Basic Body Plan
Schizocoel: Split in
the mesoderm.
Peritoneum:
(a
layer
of
mesodermal
epithelium)
lines
the body wall
of
each
compartment,
forming dorsal and
ventral mesenteries
that cover all organs
Phylum Annelida
Class
Polychaeta
Class
Clitellata
Class
Aelosomata
Aelosomata
Marine worms
Subclass
Errantia
Subclass
Sedentaria
Subclass
Oligochaeta
earthworms
Subclass
Hirudinae
Leeches
Phylogeny of Annelids
• Polychaeta is a paraphyletic class because ancestors
of the clitellates arose from within it.
• Oligochaeta and Hirudinea form a monophyletic
group called Clitellata.
– Characterized by reproductive structure called a
clitellum.
• Class Oligochaeta is a paraphyletic group because
ancestors of leeches arose from within it.
Cladogram showing Phylogeny
Class Polychaeta
Gr. polys, many, + chaite, long
hair
– Distinct head with eyes and
tentacles (Distinct)
– Most segments with
parapodia with tufts of
setae.
– Cirri/branchiae for
respiration.
– Clitellum absent
– Usually dioecious
– Examples: Nereis
Subclass Sedentaria
•Fan worms,
•Christmas-tree worms
•Spaghetti worms
•Chaetopterus
•Lug worms
Subclass Errantia
• Sand worms,
• Scale worms
• Fire worms
• palolo worms
Class Polychaeta
• Most have parapodia and gills for gaseous exchange.
– Others use the body surface.
• Circulation varies.
– In Nereis a dorsal vessel carries blood forward and
a ventral vessel carries blood posteriorly.
– Blood flows across between these major vessels in
networks around the parapodia and intestine.
• In some, septa are incomplete and coelomic fluid
serves circulatory function.
• Many polychaetes have respiratory pigments Hemoglobin, chlorocruorin or hemerythrin.
Class Polychaeta
• Largest group of annelids
• Primarily marine (Most are benthic, but some live
pelagic lives in the open seas).
• Most segments
include a pair of paddle–like
parapodia.
• Often tube-dwelling (secrete mucus/CO3
• Fertilization is external with free-swimming larva
Sub-class of Polychaeta
Feather duster worm (Sedentaria)
Christmas tree worm
(Sedentaria)
Sphagethi worm (Sedentaria)
Chaeopteris (Sedentaria)
Lugworm
(Sedentaria)
Sub-class of Polychaeta
Scale worm (Errantia)
Scale worm (Errantia)
Lugworm
(Sedentaria)
Fire worm (Errantia)
Biology of Nereis diversicolor
• It lives in U – shaped burrows where it emerges on to
the surface and extending its anterior portion to
capture prey.
• They are predatory in habit
• The body is externally and internally divided into 100
segments.
• The parapodia are rich in blood supply and used in
respiration as well as locomotion.
• Has considerable power of regeneration
• Fertilization is external.
• Closed circulatory system and excretes mainly
ammonia through the nephridia
Lugworm in burrow
Chart showing muscular
eversible pharynx
Retracted Pharynx
Everted Pharynx
The pharynx is everted by contraction of body wall muscles.
Chart showing Cirri &
Chaetae
Class Oligochaeta (Earthworm)
General characteristics:
• few setae per segment
• term “earthworm” is academically incorrect because
aquatic & parasitic forms are included
• most are monoecious, cross-fertilization
• most highly organized animals to have regeneration
• clitellum (secretes cocoon)
Aquatic Earthworms
Subclass Oligochaeta
Gk: Oligos, few; chaetae; bristles
• Lack parapodia and have few setae
• Lack the distinctive head region of polychaetes and
have no eyes.
• Scavengers that consume soil that contains organic
matter
• The ingested soil moves into a storage chamber called
the crop, then to an area called the gizzard, where
grinding action breaks down the soil particles.
Undigested material passes out the anus in a form
called castings, which are prized as soil fertilizer.
Biology of Libyodrilus
• West Africa eudrilid earthworm inhabiting all soil
types except dry and acidic soils.
• They prefer soils rich in organic matters.
• Rainy season favours distribution.
• The body is long, pointed (120-125 segments)
corresponding exactly to the number of internal septa.
• First and last segment different from other body
segments.
• 1st segment is called the peristomium, 125th segments
called pygidium.
• All segments secrete mucus, albumen etc. necessary for
cocoon formation.
Biology of Libyodrilus
The body of Libyodrilus is divided as follows:
 Pre-clitellum region: Anterior part lying between 1st
and 13th segment.
 Clitellum region: Segment 14th to 17th.
 Post-clitellum region: Remaining body part after the
clitellum from 18th to 125th.
Reproduction
• Occurs throughout the year but copulation takes place
at night when there is warmth and increase moisture.
• Fertilization is internal within the cocoon.
• Direct development, no larvae.
• Young worms lack clitellum.
Sexual Reproduction in Earthworm
Economic Importance of Earthworms
• They improve soil fertility when burrowing and
feeding thus enhancing aeration, porosity and
exhuming rich soil.
• They increase the organic content of the soil by
dragging plant materials into their burrows.
• They are used as bait for catching fishes
• Some communities feed on earthworms
• They are used for the manufacture of drugs.
Economic Importance of Earthworms
Harmful Effects
• They destroy young plants by scavenging.
• Some are secondary host for the completion of life
stages of certain parasites.
• They aid soil erosion.
Aporrectodea
turgida
Earthworm Morphology Guide
Morphology
-Number & location of GTs and TPs,
-location & shape of clitellum
Ecology
Location of burrows
L. rubellus
Common Terrestrial Oligocheates:
Earthworms
•
•
•
•
•
•
•
•
Octagonal-tail worm (Dendrobaena octaedra)
Red marsh worm (Lumbricus rubellus)
Dew-worm or night crawler (Lumbricus terrestris)
Pink soil worm (Aporrectodea rosea)
Canadian worm (Aporrectodea tuberculata)
Pasture worm (Aporrectodea turgida)
Woodland white worm (Octolasion tyrtaeum)
Red worm (Eisenia fetida )
Class Hirudinea
General Characteristics:
• mostly fluid feeders
• fresh & marine
• possess clitellum – apparent only during reproduction
• have annelid characteristics but lack setae
• true bloodsuckers have cutting plates for cutting
through tissue of the host organism
• usually attaches by posterior sucker until suitable spot is
found for attachment of anterior sucker
• salivary glands secrete anticoagulant called hirudin that
prevents clotting.
• Microsurgeons engage medicinalis to reconnect arteries
Class Hirudinea
• Excretion by metanephridia
• Swimming by vertical undulations
or loops by using its suckers to grip surfaces.
• They exhibit variation in patterns and colours.
• They are dorso-ventrally flattened.
• Most leeches are ectoparasites (haematophagus,
sanguivorous).
• Some are predacious with suckers for anchorage.
• The gut is adapted for storage of large quantities of blood.
• Mouth opens at the base of the anterior sucker, has 3 halfmoon-shaped jaws.
Class Hirudinea
• Hermaphroditic, possessing temporary clitellum
(only during breeding).
• The clitellum secretes cocoon for reception and
storage of eggs.
• No asexual reproduction and no power of
regeneration.
Class Hirudinea
• Biology
Thanks for Listening