Annelida 🪱

Basic Info:

• Early Embryonic Development: spiral/mosaic, pro

• Body Plan: bilateral, cephalization, segmentation & limbs

• “segmented worms“; 15,000 species

• all environments — marine, freshwater, terrestrial

• Segmentation: efficiency of burrowing and movement

  • NOT homologous in annelids, arthropods, and chordates

• Dates back to Ediacaran & Cambrian fossils

• Polychaeta (80%) — marine worms, sandworms, tubeworms, bristle worms, etc.

• Olichaeta — earthworms, leeches

Human Interactions:

• food for fish

• agricultural support (soil aeration, compost/vermiculture)

• medicinal leeches

• research (Great Lakes Worm Watch)

• ecosystems

Anatomy & Physiology:

• Head (prostomium + peristomium)

  • Segmentation: repeated segments w/repeated organs

• Pygidium = tail

  • hydroskeleton - muscle contractions change segmentations

• Parapodia = respiratory organ; dorsal & ventral parts

  • used for crawling, swimming, & anchoring

Digestion: complete gut

Nutrition:

• Mobile Polychaetes = predators

• Sedentary Polychaetes = filter/suspension feeder, tentacles w/mucus + cilia

• Oligochaetes = detritus

• Hirudinea = parasites

Circulation: closed; fluids transport nutrients, wastes, respiratory gases

Respiration: parapodia; diffusion across skin

Excretion: Nephridia

1. ciliated funnel pulls in waste from previous segment

2. tubules w/capillaries reabsorb water

3. empties out nephridiopore

Nervous/Sensory:

• cerebral ganglia (brain) connects to ventral nerve cord

  • giant axons increase signal speed

• Sensory Organs:

  • eyes and statocysts

  • palps and antennae

Movement:

• Parapodia = movement/anchor

• Peristalsis = circular and longitudinal muscles + anchoring w/setae

• Undulation = longitudinal muscles

Reproduction & Development:

• Asexual = budding (Polychaetes)

• Sexual = external fertilization

  • temporary sex organs; sexes separate, may change, or hermaphroditic

  • Trochophore = early larvae

Class Clitellata

• earthworms; 5,000 species; occur in moist habitats (soil/freshwater)

  • setae, less developed heads, hermaphroditic

  • clitellum - cocoon for fertilization/development

• Digestion:

  • detritivores

  • crop stores food, gizzard grinds

  • absorption occurs in intestine

• Setae: 4 pairs; bristles moved by tiny muscles, used as anchor

• Reproduction:

  • 1. opposite/ventral sides are aligned

  • 2. sperm is exchanged

  • 3. clitellum secretes cocoon

  • 4. cocoon passes forward; eggs and sperm poured into it

  • 5. fertilization occurs in cocoon

  • 6. young worms emerge (no trochophore!)

Class Hirudinea

• leeches

  • no setae, anterior + posterior suckers, mostly freshwater, clitellum only during breeding

• Nutrition:

  • ectoparasites (blood), secretes anticoagulant in saliva

• Reproduction:

  • sperm transferred by penis/hypodermic impregnation

  • temporary clitellum (cocoon laid in mud)

Misc Oligochaetes:

• Sludgeworms (Tubifex worms), giant earthworms

Marine Polychaetes

• well-differentiated head + sense organs

• parapodia = setae in bundles

• Mobile/Errant Families: predators

  • Scale Worms - flattened body w/broad scales

  • Clamworms - most abundant

  • Fire worms - hollow, brittle setae w/poisonous neurotoxins, feeds on cnidarians

• Sedentary Families: ciliary feeders, fan-like tentacles, withdraws into a tube

  • Tubeworms - burrows, cilia + mucus to feed, tubes stabilize ecosystem

  • Fan Worms - food moved to mouth by tentacles

  • Zombie (Bone-eating) Worms - only found in whale falls

Echiura (Spoonworms)

• 140 species, marine, loss of segmentation

• burrow in shells, rocks, or crevices

• ciliated grooves on proboscis

Parchment Worms

• pulls water through their parchment-like tube and filter feeds

Phylum Sipuncula (Peanut Worms)

• 250 species; benthic, tropical marine

• sedentary - lives in burrows, shells, coral

• no segmentation/setae, retractable proboscis

• trochophore larvae