Phylum Annelida

the segmented worms

Annelids - segments• Animals that have many segments

– Include earthworms, leeches and marine polychaetes.• Most annelids have chitin-reinforced bristles --setae

on each side (to provide movement).• Oligochaetes – few e.g. earthworms• Polychaetes – many bristles per segment

• Provide evolution of regional specializations where some segments undergo modification to take on new functions. e.g. leeches have suckers at both ends and polychaetes have elaborate head and segments ` closely resembling feet’.

Annelida Characteristics

Triploblastic

Cephalization

Bilateral Symmetry

Organ level of organization

Eucoelomate

Different Body Cavities

Acoelomate

Gut

ectoderm

mesoderm

endoderm

No body cavity

Different Body Cavities

PseudocoelomateHave a “false” body cavity that is surrounded on ONE side only by mesoderm

ectoderm

mesoderm

endoderm

Gut

pseudocoelom

Different Body Cavities

EucoelomateHave a “true” body cavity that is completely surrounded by mesoderm

ectoderm

mesoderm

endoderm

coelom

Gut

Annelid Characteristics

the coelom

ectoderm• is a closed, fluid filled cavity that surrounds the gut• the fluid within acts as a circulatory system• mesodermal membranes (mesenteries) suspend organs in the coelom

Annelida Characteristics

Protostome

Development is characterized by:• blastopore becomes the mouth

Mouth Formation

blastopore archenteron(primitive gut)

Future anus

mouth

Blastopore becomes the mouth and the anus forms secondarily

Annelida Characteristics

Metamerism

The body is made up of serially repeating, coordinated segments called metameres that are separated from one another by septa.

•Each metamere contains sets of repeating organs

e.g. gut, blood vessels, nerve cord, excretory organs

Metamerism

septa

FEEDING

– Digestive tract is a long tube extending from mouth to anus

– Muscular pharynx is very diversified in annelids (can include crop and gizzard similar to birds)

– Some have sharp teeth. – Leeches use pharynx to suck blood and tissues

from host – Some aquatic annelids are filter feeders.

Annelid RESPIRATION• mainly by diffusion•Aquatic annelids breath through gills (feather duster worms)

Some use diffusion through skin (skin must stay moist)• Class Polychaeta often has specialized structures for gas exchange (e.g. parapodia, gills)

INTERNAL TRANSPORT

– Closed circulatory system with 2 blood vessels running the length of their bodies (dorsal and ventral)

– In each segment, smaller ring vessels supply blood from the main vessels to organs.

– In earthworms, some of the ring vessels work like hearts (pumping blood to the rest of the body)

NERVOUS SYSTEM

– Some have well developed nervous system – Ventral nerve cord runs the length of the body in

earthworms. – Free-living marine annelids have best developed

sense organs (balance organs, chemical receptors and eyes).

– Earthworms have no specialized sensory cells.

MOVEMENT

– Two major muscle groups (longitudinal and circular)

– (Nematodes only have longitudinal muscles)

REPRODUCTION

– Most reproduce sexually – Earthworms have structure called a clitellum

which secretes a mucus ring into which eggs and sperm are released during copulation.

Annelida Characteristics

EXCRETION

Skeletal System

• fluid in coelom acts as a hydrostatic skeleton

• excretion is accomplished by organs called nephridia (singular nephridium)

Phylum Annelida

Class Polychaeta

Class Oligochaeta

Class Hirudinea

Class Polychaeta

Class Polychaeta

• Poly = Many • Chaeta = Bristles • Aquatic worms with bristles or projections on either

side of the segments

Class Oligochaeta

Class Oligochaeta

• Oligo = Few • Chaeta = Bristles

• Includes earthworms, burrowing worms

Class Hirudinea

Class Hirudinea

Leeches: _external parasites_ • Have powerful suckers at each body end• Produce an anti-coagulant (prevents blood from

clotting) so that they can continue to drink the host’s blood. • Leeches can swallow up to 10X their weight in blood

and can survive off one meal for a whole year!

• Often used for medical purposes.

Class Polychaeta

Class Polychaeta

• all marine • this class contains 2/3 of all known Annelids• have a well developed head with specialized sense organs

Class Polychaeta

• have many setae (chitonous bristles secreted by the epidermis)

(Poly = many, chaeta= setae) • these setae are arranged in bundles on paddle-like appendages called parapodia

setae

Class Polychaeta

Parapodium

The parapodia function in gas exchange, locomotion, and feeding.

setae

Class Polychaeta

Tagmatization (tagmosis)• the fusion and specialization of formerly metameric segments

Class Polychaeta

Many are filter-feeders with specialized structures

Class Polychaeta

Many are predatory with specialized structures

Class Polychaeta

Many construct their own homes out of CaCO3 or sand debris and mucous

Class Polychaeta

Reproduction• usually dioecious• no permanent sex organs; gametes are shed into coelom• fertilization is usually external• indirect development trocophore larvae

Class PolychaetaEpitoky• In some species, the rear portion of the worm (“epitoke”: carries the eggs or sperm), breaks off from the body and swims to the surface to reproduce. The remaining benthic worm (“atoke”) continues to feed and grow and will eventually produce new epitokes.

• In other species, there are pelagic individuals that resemble epitokes but the entire individual swims to the surface where the body wall ruptures, filing the water with eggs and sperm.

• To synchronize these spawnings, this swarming behavior is usually triggered by the full moon.

Ecology• Polychaetes often have effective defense strategies:

• some have tubes to hide in

• some have vicious jaws

• some have modified “stinging” setae

a fireworm

Ecology

• Some Polychaetes have a mutualistic relationship with their host

• for example, many scaleworms are found near, or in the mouth, of brittlestars, starfish, and sea urchins. • The scaleworm eats its host’s leftovers and with its vicious jaws, it will attack any predator trying to eat it’s host.

Class Oligochaeta

Class Oligochaeta

• terrestrial, freshwater and marine• have few setae (Oligo = few, chaeta = setae)• usually feed on detritus

(decaying organic matter)

• have specialized digestive system to obtain the maximum amount of nutrients out of the detritus (e.g. typhlosole, gizzard, crop…)

Class Oligochaeta

typhlosole

gut

typhlosole-• infolding of the dorsal side of the intestine• increases surface area for absorption of nutrients

Class Oligochaeta

Locomotion

Circular muscle contraction

Longitudinal muscle contraction

Class Oligochaeta

Reproduction• usually monoecious • cross-fertilize by exchanging sperm

clitellum

testis

Ecology

• Earthworms are essential soil aerators

• If all the material ever moved through earthworms was piled up, the heap would rise 30miles , more than 5 times the height of Mount Everest!!

• Worm Grunting:

stob

A saw or leaf spring of a pick-up

Class Hirudinea

Class Hirudinea

• usually freshwater but there are some marine and terrestrial species• no septa between metameres• no setae• have 2 suckers

Class Hirudinea

• have an extendable proboscis for feeding

Class Hirudinea• usually have a fixed number of segments (34)• each metamere consists of several annuli (think accordion)

1 metamere

annuli

Class Hirudinea

Locomotion

Lack septa between metameres, so they are incapable of moving like Oligochaetes.

Instead, they use their anterior and posterior suckers to move.

Class Hirudinea

Reproduction• usually monoecious • cross-fertilize by exchanging sperm

Ecology

• Although some leeches are parasitic blood suckers (can be temporary or permanent), many are predators.

Ecology

• Leeches have been used medicinally since the 19th century.

• Currently they are used to increase blood flow following reconstructive surgery

• Hirudin is a powerful anticoagulant that is found in the salivary glands of leeches

How do the Annelids fit in?

unknown common ancestor

PoriferaCnidaria

Platyhelminthes Nemertea

Nematoda Rotifera

Hirudinea

Polychaeta

Oligochaeta

protostomeeucoelomate

metamerism