Download Annelids

Phylum Annelida - Polychaetes, Earthworms, Leeches
A bilateral body in motion, with a distinct head and tail end, opens a new pathway in evolution
The parts of the body between the head and tail are now free to specialize to perform different
functions
Among the annelid worms, and the arthropods, the body becomes organized into a linear series
of identical compartments called segments
Each segment (or metamere) is separated by cross walls called septae
This process is called segmentation (or metamerism)
Segmentation probably evolved as an adaptation for burrowing through soft sediments
It gives annelid worms a remarkable ability to push their way through soil
Segmentation takes place in the mesoderm, in the body wall and in the muscles
Supply systems, like the circulatory, nervous and excretory systems, run through each segment
Segmentation evolved independently in two groups of protostomes (annelids and arthropods) and
in the deuterostomes (chordates like us!)
In both cases, the ancestral species was a burrowing aquatic worm
Advantages of Segmentation
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Segments allow for efficient locomotion over solid surfaces
Each segment contains both circular and longitudinal muscles
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Waves of muscular contraction push against the fluid-filled segments, causing them to
contract or expand
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By anchoring segments or groups of segments to the ground with special bristles called
setae, annelids can push their way through the soil
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Small stiff bristles are made of chitin ( = cellulose plus a nitrogen group)
Setae also help to anchor the worm in its burrow when it is attacked by a predator
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Segments are free to specialize
Segments are identical - if one or more are damaged, the rest may survive
Annelids have amazing powers of regeneration
Kingdom Animalia
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Animalia
> Parazoa
> Eumetazoa
– Protostomia
Spiralia
» Platyzoa – flatworms, rotifers
» Trochozoa – mollusks, annelids
Ecdysozoa
– Deuterostomia
Phylum Annelida
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8,600 species, fr Latin annelus = little ring
Ancient group, may date back to the Precambrian
Coelomate protostomes
Primarily aquatic, even “earth”worms, both fresh water and marine
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One of Linnaeus’ original taxa – Vermes (all the “wormy” animals)
Only thing they really had in common was being longer than they were wide…
Lamarck defined the Annelida, correctly linking them to mollusks (Cuvier got it wrong,
linked them to arthropods)
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Most are small (~0.5 mm), but Australian earthworms reach nine feet, record is 22 feet!!
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Closed circulatory system
Respiration via diffusion, though many aquatic forms have gills
Excretion by nephridia, one pair/segment
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Well-developed nervous system
Brain consists of several ganglia
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Smaller ganglia, along paired ventral nerve cord, control each segment
Segments contract individually in waves
Eyes, statocysts, and chemoreceptors
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Class Polychaeta - paddle worms, Nereis
Class Oligochaeta - earthworms
Class Hirudinea - leeches
Phylum Annelida - Class Polychaeta
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5,400 species, fr Latin poly = many, chaeta = bristle - tube worms, fan worms, paddle
worms, Nereis
Most primitive annelids
Mainly marine
Common and abundant (13,425 worms/m2 found in Tampa Bay sediments!!)
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Highly cephalized
Complex sensory organs
Eyes with lens and retina
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Separate sexes
External fertilization in water
Mating swarms driven by the phases of the moon
Trochophore larvae (suggests common ancestor with mollusks)
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Each segment has a pair of paddle-like appendages called parapodia
Parapodia are covered with setae (hence many bristles)
Used for swimming, crawling, burrowing
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Parapodia also provide more surface area for respiration by diffusion
Most polychaetes also have gills (very active, so need more oxygen)
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Paddleworms have jaws of chitin (pincers), carnivorous or omnivorous
Many polychaetes are filter feeders
Phylum Annelida - Class Oligochaeta
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3,100 species, oligo = few - earthworms, Lumbricus
Live in soil and in bottom sediments of fresh water habitats
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A few species have re-invaded the ocean
Extremely abundant, one meadow yielded 8,700 worms/m2
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Most earthworms are scavengers feeding on dead organic matter, mostly vegetation
(detritus - detritivores)
Fresh water forms eat detritus and protists
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Critically important in aerating the soil
22-40 metric tons of soil per hectare pass through the guts of earthworms every year
If all the dirt ever churned up by worms were stacked up, it would cover the entire land
surface of the Earth 300 miles thick, 50 times the height of Everest!!
Important in bait industry (worm ranching or vermiculture)
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Lack parapodia, streamlined body
Lack cephalization
Lack eyes, but have light sensitive cells in some segments
Why??
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Complex circulatory system, row of 5 blood vessels acts as a heart
Pharynx draws in food
Soil particles in the crop grind the food
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Reproduce asexually by transverse fission (like flatworms)
Clitellum - series of segments swollen by large mucus glands
Hermaphroditic, fertilize one another simultaneously
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Clitellum secretes mucus to hold worms together while they mate
Fertilized eggs released into mucus
Mucus dries into protective cocoon
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Phylum Annelida - Class Hirudinea
500 species - leeches
Most modified as parasites, some are scavengers or predators, feeding on worms, snails,
insects
75% of species suck blood from mammals or crustaceans
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Common in fresh water habitats, a few species are marine or terrestrial
Can be abundant - one stream in Illinois had 10,000 leeches/m2 !!
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Highly modified as parasites
Store blood meal in special pouches on the digestive tract
Leeches suck up to 5 to 10 times their own weight in blood each time they feed
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Anterior and posterior sucker, attach to host
Suckers also help them crawl across the bottom
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Coelom is greatly reduced, not divided into compartments like other annelids
Leeches are strong swimmers, no longer need the adaptations of a burrowing animal
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Bite of the leech is antiseptic
Leeches release an anticoagulant in their saliva, keeps the blood flowing
For centuries doctors used leeches to bleed their patients (often to death) to get rid of “bad
blood” (imbalance of humours)
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At height of the leech craze in the 1830’s, France was importing over 50 million medicinal
leeches per year
Medicinal leech is now a threatened species!
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Medicinal leeches (Hiruo medicinalis) undergoing a revival
Used to drain blood clots and to drain postoperative swelling, bleeding
Unlikely to evolve resistance - secretions go after seven critical steps in the clotting
process!
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The traditional remedy is to pour salt on a feeding leech, or touch it with a burning
cigarette - don’t pull it off!
Recent studies show that these techniques cause the leech to vomit into the wound before
letting go – enhances the risk of infection – peel them off gently with fingernails or a dull
butter knife!
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One species of leech (Placobdelloides jaegerskioeldi) has one of the most unusual habitats
of any annelid
It lives and mates only in the rectum of the hippopotamus!
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Annelids share a common ancestor with mollusks
Polychaetes and earthworms probably evolved from primitive burrowing marine worms
Leeches and earthworms are sister clades, polychaetes are bit of a mystery…
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Leeches probably evolved from earthworms
> Lack parapodia and cephalization
> Hermaphroditic, develop a clitellum to breed
> Lay eggs in a cocoon