ZLY 103Animal Diversity

Phylum Annelida

IntroductionPhylum 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

IntroductionThey 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 ContributionsAnnelids opened a wide complexity and

variability in forms and structure, viz:-Metamerism (Segmentation) foundation for

specializationPresence 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-redundancy4. 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 Inclusion1. 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

• septumone metamere

l.s. segmented worm

Body Cavities

Mesoderm

Ectoderm

Endoderm

coelom

gut

EucoelomatesBody cavity completely lined with mesoderm

Basic Body Plan

Schizocoel: Split in the mesoderm.Peritoneum:(a layer of mesodermal epithelium) lines the body wallof each compartment, forming dorsal and ventral mesenteriesthat cover all organs

Phylum Annelida

SubclassOligochaeta

Class Polychaeta

Class Clitellata

ClassAelosomata

Marine worms

Leeches

Aelosomata

SubclassHirudinae

earthworms

Subclass Errantia

Subclass Sedentaria

• 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.

Phylogeny of Annelids

Cladogram showing Phylogeny

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

Class Polychaeta

Sub-class of Polychaeta

Feather duster worm (Sedentaria)

Christmas tree worm (Sedentaria)

Lugworm (Sedentaria)

Chaeopteris (Sedentaria)

Sphagethi worm (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

The pharynx is everted by contraction of body wall muscles.

Retracted Pharynx

Everted Pharynx

Chart showing muscular eversible pharynx

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.

Earthworm Morphology GuideMorphology-Number & location of GTs

and TPs, -location & shape of clitellum

EcologyLocation of burrows

L. rubellus

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• 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 )

Common Terrestrial Oligocheates: Earthworms

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 half-moon-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

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