Annelids – Part 1Annelids – Part 1

AKA Segmented RoundwormsAKA Segmented Roundworms

AnnelidsAnnelids

• The word annelid is derived from The word annelid is derived from the word annulus which means the word annulus which means ring.ring.– In this way, annelids get their name In this way, annelids get their name

from their most defining characteristic from their most defining characteristic – their external and internal – their external and internal segments, or rings.segments, or rings.

Classes of AnnelidsClasses of Annelids

• Annelids have three main classes Annelids have three main classes based on habitat and differences in based on habitat and differences in body structure.body structure.

• They are:They are:– Polychaete (Polychaete (paulpaul-ee-keet)-ee-keet)– Oligochaete (Oligochaete (awlawl-ee-go-keet)-ee-go-keet)– Hirudinea (hir-Hirudinea (hir-oooo--dindin-ee--ee-uhuh))

Polychaete WormsPolychaete Worms

• AKA clamwormsAKA clamworms• Size: millimeters to Size: millimeters to

meters long meters long • Change in body Change in body

structure: has many structure: has many bristles.bristles.

• Habitat: marine Habitat: marine environmentenvironment

• Types of movement: Types of movement: sedentary or mobilesedentary or mobile

Mobile PolychaeteMobile Polychaete

• Mobile worms:Mobile worms:• Tend to swim around and Tend to swim around and

catch foodcatch food• Have parapodia (little flaps Have parapodia (little flaps

of skin that act as legs) for of skin that act as legs) for movement.movement.

• Parapodia have setae Parapodia have setae (bristles) and cirri (needle-(bristles) and cirri (needle-like projections) on them.like projections) on them.

• Have eyes, sensory Have eyes, sensory tentacles, and a tentacles, and a retractable pharynx with retractable pharynx with jaws.jaws.– Ex. Ex. Nereis virensNereis virens

Sedentary PolychaeteSedentary Polychaete

• Sedentary WormsSedentary Worms• Tend to burrow in sand or Tend to burrow in sand or

mud & filter feed.mud & filter feed.• They suck up sand or mud They suck up sand or mud

and selectively absorb and selectively absorb nutrients.nutrients.

• Often lack eyes and have Often lack eyes and have much smaller parapodia.much smaller parapodia.

• Some secrete their own tube Some secrete their own tube to burrow into.to burrow into.

• Ex. Ex. ArenicolaArenicola

Oligochaete WormsOligochaete Worms

• Change in body structure: few Change in body structure: few bristles.bristles.

• Habitat: damp terrestrial Habitat: damp terrestrial environmentsenvironments

• Best known oligochaete is the Best known oligochaete is the earthwormearthworm

EarthwormEarthworm

• The earthworm is mostly nocturnal The earthworm is mostly nocturnal and tends to live in underground and tends to live in underground tunnels.tunnels.

• They help to develop and add They help to develop and add nutrients to the soil.nutrients to the soil.

• They are considered to be more They are considered to be more advanced than polychaete.advanced than polychaete.

HirudineaHirudinea

• Change in body structure: flattened Change in body structure: flattened body, with no parapodia or bristles.body, with no parapodia or bristles.

• Habitat: freshwater or terrestrial Habitat: freshwater or terrestrial parasites or predators.parasites or predators.– They attach to their prey by suckers.They attach to their prey by suckers.

• Ex. LeechesEx. Leeches– After having it’s fill of a victims blood they After having it’s fill of a victims blood they

can go for weeks or months without eating.can go for weeks or months without eating.

Annelid Body StructureAnnelid Body Structure

• Bilateral symmetryBilateral symmetry• Coelomates: have a lined coelom Coelomates: have a lined coelom

(body cavity). (body cavity). – This is an advancement over This is an advancement over

nematodes.nematodes.• Cuticle covered body to keep the Cuticle covered body to keep the

worm from drying out, while still worm from drying out, while still allowing movement.allowing movement.

Digestive SystemDigestive System• More advanced than the nematodes.More advanced than the nematodes.• One way system, in addition to the One way system, in addition to the

nematode parts they have:nematode parts they have:– Calciferous glandsCalciferous glands

• Monitor acid-base levels of the Monitor acid-base levels of the coelomic and blood fluids.coelomic and blood fluids.

– CropCrop• Used to store food.Used to store food.

– GizzardGizzard• Food is ground into digestible pieces.Food is ground into digestible pieces.

Mouth Mouth buccal cavity buccal cavity pharynx pharynx esophagus esophagus (attached to calciferous glands) (attached to calciferous glands) crop crop gizzard gizzard intestine intestine anus anus

Nervous SystemNervous System

• Anterior brain with paired ventral nerve Anterior brain with paired ventral nerve cords.cords.– Has peripheral nerves that supply the Has peripheral nerves that supply the

muscle layers and reach the sense receptors muscle layers and reach the sense receptors of the outer wall.of the outer wall.

• Also have ganglia that allow for movement.Also have ganglia that allow for movement.• Each segment has a nerve cord that passes Each segment has a nerve cord that passes

through a ganglia and attaches to sense through a ganglia and attaches to sense receptors.receptors.– This nerve cord consists of giant nerve fibers This nerve cord consists of giant nerve fibers

and is responsible for coordinating and is responsible for coordinating movement of all the segments.movement of all the segments.

Muscular SystemMuscular System

• Both longitudinal and Both longitudinal and circular muscles – circular muscles – another advancement another advancement over nematodes.over nematodes.

• Seta retractors are Seta retractors are muscles that connect muscles that connect from one set of setae to from one set of setae to another. This allows for another. This allows for coordinated movement.coordinated movement.– Remember, setae are Remember, setae are

bundles of bristles.bundles of bristles.

Circulatory SystemCirculatory System

• We haven’t seen one of these since We haven’t seen one of these since the piggy!the piggy!

• Two kinds are seen in annelids:Two kinds are seen in annelids:– OpenOpen: no vessels, blood empties into : no vessels, blood empties into

sinuses, or cavitites. (in the leech)sinuses, or cavitites. (in the leech)– ClosedClosed: has vessels : has vessels to hold and direct to hold and direct blood. (in the clam-blood. (in the clam- worm & earthworm)worm & earthworm)

ReproductionReproduction

• Annelids are hermaphroditic.Annelids are hermaphroditic.• They reproduce sexually.They reproduce sexually.

Excretory Excretory SystemSystem

The excretory system is called a The excretory system is called a metanepridium.metanepridium.– Each segment has a funnel like nephrostome Each segment has a funnel like nephrostome

which is covered by tiny hair-like structures which is covered by tiny hair-like structures (cilia) leading into the coelom of that segment. (cilia) leading into the coelom of that segment.

– The segments are connected together by a The segments are connected together by a convuluted tubule which leads from the convuluted tubule which leads from the nephrostome into the bladder of another nephrostome into the bladder of another segment. segment.

– Wastes are excreted through the nephridiopore.Wastes are excreted through the nephridiopore.

A note on leechesA note on leeches

• Leeches are sometimes considered to be Leeches are sometimes considered to be parasitic organisms. parasitic organisms.

• However, their relationship with the host However, their relationship with the host is relatively brief, so it is more correct to is relatively brief, so it is more correct to classify them as predatory.classify them as predatory.– #13 on page 99 in your resource book #13 on page 99 in your resource book

incorrectly states that they are not free-incorrectly states that they are not free-living, but rather are considered to be a living, but rather are considered to be a parasitic organism.parasitic organism.

Annelid ImportanceAnnelid Importance

• Annelids are important to humans Annelids are important to humans for the following reasons:for the following reasons:– Earthworms help to add nutrients to Earthworms help to add nutrients to

our soil.our soil.– Leeches are used medicinally to help Leeches are used medicinally to help

promote blood flow to areas.promote blood flow to areas.– They make good bait for fishing!!They make good bait for fishing!!