Phylum Annelida

Phylum Annelida Greek + Latin roots

• Annel

  • Annelus = little ring (Latin)

• -ida

  • Eidos = form, shape (Greek)

• little ring form, referring to their shape

Annelida place in Tree of Life

• part of larger group of Lophotrochozoa

  • protostomes including Platyhelminthes, Mollusca, Annelida, + many smaller groups

Annelida basics

• >15,000 species

• live in marine, freshwater, + moist terrestrial habitats

  • 2/3 are marine (polychaetes)

• metamerism

• most have tiny chitinous bristles (setae)

• include earthworms, leeches, + marine bristleworms

metamerism

• each body segment of an annelid is a repeated unit (metamere)

• each metamere contains:

  • circulatory system

  • nervous system

  • excretory system

Annelid coeloms

• fluid-filled coelem develops during embryo as a split in mesoderm

• works as hydrostatic skeleton to support antagonistic muscle action

• body wall surrounding peritoneum + coelom has strong muscles for swimming, crawling, burrowing movements

peritoneum

layer of mesodermal epithelium lining annelid body wall

annelid body plan: head + terminal area

• head = 2 parts:

  • prostomium (before the mouth)

  • peristomium (around mouth)

• terminal area = pygidium

  • includes the anus

annelid prostomium

• in front of (but doesn’t include) the mouth

• small shelf-like extension over the dorsal side of mouth

• contains brain + sensory structures like the eyes, antennae, + sometimes the palps

annelid peristomium

• behind prostomium

• contains mouth, pharynx, tentacles/cirri, + sometimes the palps

annelid body plan: segmented body

• each segment has respiratory, nervous, + excretory structures

• segments develop from in front of the pygidium

• areas near terminal end are younger than those near the head

Annelida: Errantia group

• motile + mostly marine polychaetes

• free-moving worms

• include clam worms + Bobbit worms

• can be bright, luminescent colors

• euryhaline (can withstand broad range of salinity)

• many live under rocks, in coral crevices, or in empty shells

• important food sources for fish, crustaceans, hydroids, etc

Errantia parapodia

• special sense organs

  • paired (left + ride sides of body)

  • may have lobes, cirri, setae, etc

  • used in crawling motions

  • might be used in respiration, unless species has gills

Errantia polychaete feeding

• predators + scavengers

• digestive system includes:

  • foregut: mouth, pharynx, esophagus

  • midgut: secrete digestive enzymes, absorption

  • hindgut: connects midgut to anus

Errantia nervous system

• highly developed sense organs

• have eyes, nuchal organs, + statocysts

  • nuchal organs = ciliated sensory pits w/chemoreception

  • statocysts = help w/body orientation

Errantia reproduction

• usually dioecious (separate sexes, not hermaphroditic)

• no permanent sex organs

  • gonads swell from peritoneum + shed their gametes into coelom

  • gametes released thru gonoducts via metanephridia

Errantia seasonal sexual maturity

• some polychaetes exist as sexually immature atokes most of the time

• during breeding season, posterior portions mature w/gametes (epitokes)

• epitokes separate + swim to surface of water w/other epitokes + externally fertilize by bursting to release eggs + sperm

• other species asexually bud off + posterior end becomes multiple sexual epitokes

Annelida: Sedentaria group

• non-motile

• mainly found in low-oxygen marine habitats

• polychaetes + oligochates that live in burrows

• many tube-dwellers have gills

• have hooklike setae to grasp onto tube walls

• include beardworms + fanworms

Sedentaria body plan

• similar to Errantia, but head is often modified w/tentacles

• tentacles used by all Sedentaria for capturing food

  • some Sedentaria use tentacles for respiration as well

Phylum Annelida: Clitellata

• lack parapodia (major difference from Errantia + Sedentaria)

• all are monoecious (hermaphroditic)

• young develop in cocoons

• emerge as juvenile worms

  • no visible trochophore stage

Clitellata phylogeny

• in Phylum Annelida

• monophyletic grouping

• united by having a clitellum

  • ring of secretory cells in epidermis, sometimes only showing up during breeding season

Clitellata movement mechanisms

• use hydraulic pressure from fluid in coeloms for movement

• coelomic fluid acts as hydrostatic skeleton

• fluid is incompressible

• move fluid to different segments to create areas of high internal fluid pressure, creating rigidity

Clitellata peristaltic movement

• constriction + relaxation using longitudinal + circular muscles

  • perpendicularly located to each other

• longitudinal muscles contract

  • make segments short + wide to push against a burrow

  • setae project outwards to hold the segments

• then circular muscles contract

  • extends the body

Clitellata feeding

• scavengers + decomposers

  • not predators or parasites like other annelids are

• feed on decaying organic matter, leaves, + dead animals

Clitellata feeding process

• mouth secretes fluids to moisten food

• pharynx sucks food inwards

• passes thru esophagus

• stored in crop

• ground up in gizzard (used to fill role of teeth for grinding food)

• digested + absorbed in intestine

• excreted by anus (separate mouth + anus, aka complete gut)

Clitellata esophagus

• used to regulate pH + calcium levels after eating food

• passes thru aortic arches but doesn’t mix contents w/them

  • both it + the aortic arches are separate structures

Clitellata intestines

• very long intestines = max surface area

• intestinal wall has typhosole

  • extra infolding

  • increases intestinal surface area for max nutrient absorption

Clitellata circulatory system

• closed circulatory system where blood circulates

• food, waste, + respiratory gases are carried by both coelomic fluid + blood

Clitellata circulatory system vessels

• dorsal vessel:

  • functions like a heart

  • pumps blood from body wall + digestive tract into 5 aortic arches

• ventral vessel:

  • receives blood from arches

  • delivers it to the rest of body, including brain

Clitellata respiration

• earthworms have no special respiratory organs

• gas exchange occurs across epidermis 

  • bc of that, they prefer moist environments

Clitellata excretory system

• each segment has pair of metanephridia

• cilia bring waste from coelom to nephrostome

• more waste from blood capillaries added to tubule

• wastes excreted by nephridiopore

aquatic vs terrestrial oligochaete excretion

• aquatic excrete ammonia

• terrestrial excrete urea or both ammonia + urea

  • many animals invest energy to transform toxic ammonia into less harmful nitrogenous compounds

Clitellata nervous system

• central nervous system + peripheral nerves

• cerebral ganglia = brain, which is connected to:

  • ventral nerve cord in coelom extending to last segment

  • pair of fused ganglia per segment

• no eyes, photoreceptors in skin

• chemoreceptors especially in prostomium

Clitellata reproduction

• sexual reproduction only (NO asexual)

• reproduction occurs year-round

• monoecious (hermaphroditic)

  • 2 pairs of testes produce immature sperm

  • 3 pairs of seminal vesicles (where sperm mature)

Clitellata reproduction process

• mating worms bring their ventral surfaces together

• sperm travels to seminal receptacles of other worm thru seminal grooves (guide transfer of sperm)

• worm secretes mucous tube + chitinous cocoon from clitellum that slides forward

  • eggs from oviduct, albumin from skin glands, + sperm from seminal receptacles all move into cocoon

Clitellata fertilization

• external fertilization happens in cocoon

• cocoon continues to slide off annelid’s body

• cocoon ends seal

• embryogenesis happens in cocoon

• juvenile worms emerge later

• clitella develop in sexually mature worms

Phylum Annelida: Hirundinida

• leeches

• occur mostly in freshwater habitats, but some live in moist terrestrial habitats

• more common in tropical areas

• usually flattened dorsoventrally

• generally carnivorous or parasitic

Hirundinida segments

• fixed number of segments

• each segment has transverse grooves, which look like rings

• 3 classes:

  • Hirudinea = 34 segments

  • Acanthobdellida = 27 segments

  • Branchiobdellida = 14/15 segments

Hirundinida suckers

• have anterior + posterior sucker

• use them for attachment, locomotion, + feeding

• capable of inching by suctioning movements + dragging body along surface

Hirundinida feeding habits

• ~25% predatory

• ~75% parasitic

• can force proboscis (pharynx) into soft tissues or to take blood

• might use oral sucker to hold prey down while pharynx ingests them

Hirundinida fluid feeding

• most leeches primarily fluid feeders

• true bloodsuckers have chitinous jaws that cut thru tough skin

• have salivary glands that produce an anesthetic + anticoagulant

• most don’t stay on their hosts for a long time