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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 • • Segments allow for efficient locomotion over solid surfaces Each segment contains both circular and longitudinal muscles • Waves of muscular contraction push against the fluid-filled segments, causing them to contract or expand • By anchoring segments or groups of segments to the ground with special bristles called setae, annelids can push their way through the soil • • 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 • • • 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 • Animalia > Parazoa > Eumetazoa – Protostomia Spiralia » Platyzoa – flatworms, rotifers » Trochozoa – mollusks, annelids Ecdysozoa – Deuterostomia Phylum Annelida • • • • 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 • • • 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) • Most are small (~0.5 mm), but Australian earthworms reach nine feet, record is 22 feet!! • • • Closed circulatory system Respiration via diffusion, though many aquatic forms have gills Excretion by nephridia, one pair/segment • • Well-developed nervous system Brain consists of several ganglia • • • Smaller ganglia, along paired ventral nerve cord, control each segment Segments contract individually in waves Eyes, statocysts, and chemoreceptors • • • Class Polychaeta - paddle worms, Nereis Class Oligochaeta - earthworms Class Hirudinea - leeches Phylum Annelida - Class Polychaeta • • • • 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!!) • • • Highly cephalized Complex sensory organs Eyes with lens and retina • • • • Separate sexes External fertilization in water Mating swarms driven by the phases of the moon Trochophore larvae (suggests common ancestor with mollusks) • • • Each segment has a pair of paddle-like appendages called parapodia Parapodia are covered with setae (hence many bristles) Used for swimming, crawling, burrowing • • Parapodia also provide more surface area for respiration by diffusion Most polychaetes also have gills (very active, so need more oxygen) • • Paddleworms have jaws of chitin (pincers), carnivorous or omnivorous Many polychaetes are filter feeders Phylum Annelida - Class Oligochaeta • • 3,100 species, oligo = few - earthworms, Lumbricus Live in soil and in bottom sediments of fresh water habitats • • A few species have re-invaded the ocean Extremely abundant, one meadow yielded 8,700 worms/m2 • Most earthworms are scavengers feeding on dead organic matter, mostly vegetation (detritus - detritivores) Fresh water forms eat detritus and protists • • • • • 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) • • • • Lack parapodia, streamlined body Lack cephalization Lack eyes, but have light sensitive cells in some segments Why?? • • • Complex circulatory system, row of 5 blood vessels acts as a heart Pharynx draws in food Soil particles in the crop grind the food • • • Reproduce asexually by transverse fission (like flatworms) Clitellum - series of segments swollen by large mucus glands Hermaphroditic, fertilize one another simultaneously • • • Clitellum secretes mucus to hold worms together while they mate Fertilized eggs released into mucus Mucus dries into protective cocoon • 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 • • Common in fresh water habitats, a few species are marine or terrestrial Can be abundant - one stream in Illinois had 10,000 leeches/m2 !! • • • 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 • • Anterior and posterior sucker, attach to host Suckers also help them crawl across the bottom • • Coelom is greatly reduced, not divided into compartments like other annelids Leeches are strong swimmers, no longer need the adaptations of a burrowing animal • • • 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) • 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! • • • • • • 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! • 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! • • • 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! • • • 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… • Leeches probably evolved from earthworms > Lack parapodia and cephalization > Hermaphroditic, develop a clitellum to breed > Lay eggs in a cocoon