PHYLUM ANNELIDA

The Segmented Worms

Characteristics

• Bilateral symmetry• Body metamerically segmented• Chitinous setae (bristles) often present on

parapodia• Digestive system complete• Respiratory gas exchange through skin, gills or

parapodia• Closed circulatory system• Excretory system typically a pair of nephridia

for each segment

Characteristics

• Coelom well developed and divided by septa (except in leeches); coelomic fluid functions as hydrostatic skeleton– Coelom develops embryonically as a split in the

mesoderm on each side of the gut (schizocoelomate)

• Body wall with outer circular and inner longitudinal muscle layers

• Transparent moist cuticle secreted by epithelium• Hermaphroditic or separate sexes

– Larvae if present are trochophore– Asexual reproduction by budding in some– Spiral, determinate cleavage of the embryo

Polychaete

Body Plan• 3 regions:

– Head – Prostomium & peristomium– Segments (metameres)– Pygidium• New segments formed at the posterior end

Body planMetamerism

• Divides body into linear series of segments (metameres) separated by septa

• Repetition of organs, tissues and appendages in each segment

• Allows for regional specialisation

• Regional specialisation can result from:1. Restriction of certain structures to only a few

segments

2. Structures retained in all segments, but diverge and adopt different functions

3. Fusion of segments

Body planMetamerism

• Contraction of circular or longitudinal muscle causes a segment to become either longer and thinner or shorter and wider, respectively

• Movement achieved by alternating waves of contraction of circular and longitudinal muscle (peristaltic contraction)

Body planMetamerism

Circulatory System

• Circulatory system closed• Respiratory pigments often present• Aortic arches (“hearts”) for pumping blood

Nervous System

• Pair of cerebral ganglia (brain)

• Two fused ventral nerve chords

• Lateral nerves in each segment

Nervous System

• Giant axons in ventral chord conduct impulses very quickly

• Sensory system of tactile organs, taste buds, statocysts (in some), photoreceptor cells and eyes with lenses (in some)

Excretory System

• Pair of nephridia in each metamere (except first 3 and last 1)

Excretory System

• Each nephridium occupies 2 adjacent segments

• Nephrostome (internal opening) leads via small ciliated tubule to segment behind where it connects with main part of nephridium

• Nephridium made of several loops of tubule that terminate in a nephridiopore (external opening)

Excretory System

• Cilia draw fluid from the coelom into the nephrostome

• Water and salts reabsorbed

• Urine released through the nephridiopore

Reproduction

• Monoecious or dioecious• Larvae (if present) are

trochophore type– Trochophore: Free-swimming,

translucent, marine larva with an ovoid body and ring of cilia

• Asexual reproduction by budding in some

Polychaete

Class: Polychaeta

• Largest class• Mostly marine• May live under rocks or in crevices,

burrow into sand, build tubes, or be pelagic

• Well differentiated head with specialised sense organs

• No clitellum

Free-moving sedentary

Class: Polychaeta

• Paired appendages (parapodia) on most segments – Used for locomotion and respiration

• Many setae, usually arranged in bundles on the parapodia

Class: Polychaeta• Two groups:

– Sedentary polychaetes • Spend most of their time in

tubes or permanent burrows• Ciliary feeders with tenticular

crown

– Errant polychaetes • Include free-moving pelagic

forms, active burrowers and crawlers, and tube worms that leave their tubes for breeding and feeding

• Predatory forms with pharynx, teeth and jaw

Free-moving

sedentary

sedentary

Class: Polychaeta

Reproduction:• No permanent sex organs

or ducts for sex cells• Sexes usually separate • External fertilization• Indirect development with

trochophore larvae• Asexual budding in some

prototroch

mouth

anus

Class: Oligochaeta

• Earthworms• Mostly terrestrial or freshwater,

some parasitic and a few live in marine or brackish water

• Few setae per segment• No parapodia• Head absent

Class: Oligochaeta

• Typhlosole increases surface area for absorption of nutrients

Reproduction in Earthworms

• Hermaphroditic – copulate to exchange sperm• Copulating worms held together by mucous

secreted by clitellum and by special ventral setae– Clitellum develops only in sexually mature

worms• After copulation, each worm forms a cocoon

around its clitellum, which slides forward along the body

Reproduction in Earthworms

• As the cocoon moves along the body, eggs and sperm are released into it and fertilization takes place inside the cocoon

• Cocoon eventually slides off the head of the worm and its ends close up

• Young emerge from the cocoon as small worms- direct development

Class: Hirudinea

• Leeches

• Mostly freshwater, some marine or terrestrial

• Dorsoventrally flattened

• Usually both anterior and posterior sucker

• No parapodia

• Usually no setae

Class: Hirudinea

• Coelom filled with connective tissue and muscle – does not function well as hydrostatic skeleton

• Many are carnivores of small invertebrates, some temporary parasites and some permanent parasites

• Feed on body juices of prey – have a muscular pharynx and three jaws with teeth

Class: Hirudinea

• Fixed number of segments (34) that are not usually divided internally by septa

• Hermaphroditic • Clitellum appears only during breeding

season• Direct development

Evolutionary Relationships

Evolutionary Relationships

• Annelids most closely related to either arthropods or molluscs

• Share with arthropods a segmented body plan and a similar nervous system

• Share with molluscs similar features of early embryology and a trochophore larva