Annelids – Defining Characteristics

Segmented wormsCoelomatesTrochophore larvaeprotostome

Segmentation

Annelids – FeedingFilter feeders – use gills to capture food particles from waterOthers use pharynx to either suck in food or to extend out and get it. Some annelids have jaws on their pharnyxChristmas tree worm

Nereis – (clam worm) -has jaws for eating and fighting

Respiration

Aquatic annelids use gillsTerrestrial annelids exchange gases through their skin – their skin must remain moist for this purposeCuticle on the earthworm helps prevent desiccation

Feather-duster worm – gills exposed (red)- tentacles for gathering food

Earthworm cuticle

Internal Transport

Closed circulatory systemDorsal surface has blood moving toward the head, ventral surface moves blood away from the headEach segment has a ring vessel connecting the ventral with the dorsalLarge ring vessels at anterior end act as a “heart” to pump the bloodIf no heart is present, the bodies muscular contractions move the blood around

Excretion

Solid wastes out the anusMetabolic (liquid) wastes are eliminated by nephridia (like the nephrons in our kidneys)2 nephridia/segment

Response

Brain on dorsal surfaceNerves travel from the brain, around the gut and a main nerve cord travels down the ventral surfaceBrain = #11

Ganglia = #12Ventral Nerve cord = #13

Sense organs Best developed in free living annelidsMost of these have sensory tentacles, statocysts, chemoreceptors, and “eyes”Some form images, some are light-detectorsSome have sensory organs that detect vibrations (like our tympanic membranes)

Spaghetti worm undergroundWith tentacles exposed

Self Defence

Burrow into ground or swim awaySetae (bristles) anchor worms in the soilMarine fan worms make tubes of CaCO3 to hide in

Tube worms

Self-Defence Marine fireworms have poisonous bristles (burns)Carnivorous annelids have jaws to bite their predators

Movement

longitudinal muscles shorten the bodyCircular muscles contract to decrease the diameter of the wormCoordination of these sets of muscles allow the worms to wiggle, burrow and swim

Reproduction

Some reproduce by buddingSome have separate sexes and have external fertilizationPalolo worms congregate at the surface and release sperm or eggs at the same timeFeast for the predators (humans included!)

Earthworms are hermaphrodites and exchange spermSperm is stored in sacs until eggs are readyMucus ring is secreted by the clitellum – sperm and eggs are deposited there and the ring slips off the worm, forming a cocoon for the fertilized eggs

Common Characteristics in all Annelids

Unidirectional Digestive SystemClosed Circulatory System First sign of a ‘heart-like’ structure

Nephridia used for excretionPrimitive Brains present

Class Polychaeta – bristleworms

Mainly marine wormHave padddle-like appendages with bristlesShow cephalization

More Polychaetes

Polychaetes

Class Oligochaeta - Earthworms and RelativesEarthworms + Freshwater wormsAquatic – eg tubifex (red, thread-like worms sold for fish food)Oligo – means few (as in few bristles)Castings are excellent fertilizer

Class Hirudinea

Commonly known as the LEECHESFresh and saltwater species (~300)Few land speciesMuscular proboscis is forced into skinOthers slice skin with their jaws

Leeches

Hirudin – anti-coagulating chemical is used to keep host’s blood flowingSome also use an aesthetic so the host does not know they are thereUsed for medicinal purposes

Annelids aerate the soil by burrowingReturn nutrients to the soil completing decomposition of materialMany annelids are the basis of food chains

Internal Anatomy

Identifying practice: