Phylum Annelida

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Phylum Annelida

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the segmented worms

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Characteristics of Phylum Annelida

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consists of marine polychaetes, eathworms, leeches, & marine "beardworms"; bilaterally symmetric; organ-level of organization; terrestrial, marine, & freshwater organisms; swim, burrow, or crawl into a substrate; predators or scavengers; some are ectoparasites; protosostomes and lophotrochozoans; coelomates; schizocoelus coelom formation

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nutrient redistribution

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conversion of organic debris into CO2, which dissolves in water and is taken up by marine phytoplankton for photosynthesis, releasing O2; annelids are important in marine ecosystems for this reason

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Earthworms are important in terrestrial ecosystems for.....

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redistributing nutrients, soil drainage, and aeration

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metameric

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body plan with a series of repeating segments called metameres

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metamerism

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division of the body into distinct, serially repeated coelomic segments or "compartments" (metameres)

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annuli

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external circular rings separating metameres

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septa

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internal separation of metameres; components of major body systems (circulatory, excretory, reproductive, & nervous) repeat in mostly each segment, the gut is the only exception (it runs all the way through segments)

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evolutionary significance of metamerism

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permits greater flexibility, complexity, and larger size leading to greater species diversity; now a need for a closed circulatory system to traverse the segments

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closed circulatory system

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dorsal vessel, ventral vessel, & capillary beds; dorsal vessel pushes blood forward (anteriorly), ventral vessel moves blood posteriorly, capillary beds connect the two major blood vessels in the body wall (where blood gets oxygenated by diffusion) and in the gut wall (where blood picks up nutrients)

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cuticle

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a moist, acellular collagen secreted by epidermal epithelial tissue

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setae

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tiny chitinous bristles ( all annelids except leeches have these); short, stiff bristles anchor a segment in an earthworm and prevent slippage backward; long bristles help aquatic annelids swim

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Roles of coelom in annelids

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serves as a hydrostatic skeleton; role for transporting nutrients and metabolic wastes is now diminished

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Annelid movement

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circular & longitudinal muscles in the body wall allow for independent movement of metameres, resulting in more complex and variable forms of locomotion

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parapodia

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paired "paddle-like appendages" in marine polychaetes; sometimes have setae on them

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aciculum

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chitinous support rod in a parapodium with oblique muscles attached

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Burrowing annelid movement

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alternative waves of relaxation & contraction of circular & longitudinal muscles (similar to peristalsis)

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Annelid nervous system

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includes a pair of cephalic ganglia (brain) attached to a double ventral nerve cord that runs the length of the animal with additional ganglia and nerve branches repeated in each segment; some combinations of tactile organs, chemoreceptors, balance receptors, and photoreceptors; some predatory and swimming forms have fairly well developed eyes, including lenses

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Annelid excretory system

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increased activity & movement increases metabolism; this requires a more efficient means to deal with metabolic waste (more elaborate/efficient excretory system)

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metanephridia tubules

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in almost all segments collect & filter fluid, removing nitrogenous wastes (urea in terrestrial species, ammonia in aquatic species) to outside through nephridiopores

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gas exchange in annelids

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occurs by diffusion across the body wall, modified to "gills" in some species; large annelids may use respiratory pigments to increase the oxygen-carrying capacity of blood

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Annelid digestive tract

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annelids have a complete digestive tract; the gut is often modified into specialized regions for different tasks (buccal cavity, pharynx, esophagus, crop, gizzard)

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crop

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temporarily stores food

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buccal cavity

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space between the cheeks & jaws

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gizzard

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grinds up food

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typhlosole

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a U-shaped intestine found in earthworms to increase surface area for absorption

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Annelid feeding

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many modes: filter feeders, carnivores, herbivores, deposit feeders, blood-sucking parasites

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protrusible pharynx

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long, muscular pharynx carnivores and herbivores have for feeding

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ciliated tentacles

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coated with mucus to trap and transport food particles to mouth (found in surface deposit and filter feeders); subsurface deposit feeders may also have a short, sac-like pharynx

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Annelid reproduction & early development

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monoecious & dioecious species; oligochaetes and leeches are monoecious; development is either direct (young adult emerges) or indirect (with a ciliated trochophore larval stage); fertilized egg undergoes spiral cleavage and blastula develops into a ciliated trochophore larva with three body parts (prototroch, metatroch, telotroch); as the larva grows, segments are added between the metatroch and telotroch

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Annelid growth & development

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adult develops as segments added to larva; the anterior "head" of adult is comprised of the prostomium and peristomium; the terminal "tip" bearing the anus is the pygidium; prostomium, peristomium, & pygidium are not considered metameres like the other segments; new metameres form just in front of the pygidium (the newest segments are at the posterior end)

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Annelid Taxonomy

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Class Polychaeta
Class Hirudinea
Class Oligochaeta

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Class Polychaeta

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mainly marine and usually benthic (in or on substrate); now inlcude beardworms; most ancient; metameric with bilobed parapodia & setae on each segment; some exhibit epitoky ( no permanent gonads, gonads form only during the breeding season and usually in posterior segments); others are dioecious(male and female gametes are released via temporary gonoducts, nephridia, or by segments rupturing); externally fertilized eggs develop into ciliated trochophore larva (indirect development)

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Class Oligochaeta

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mostly in moist terrestrial habitats (ex: earthworms), some freshwater species; sometimes put into new class Clitellata; monoecious worms that cross-fertilize, a secretory clitellum wraps eggs in a cocoon where eggs develop directly into small worms; have short, stiff setae to anchor and gain traction; segments separated by external annuli and internal septa; primarily deposit or substrate feeders, a few aquatic forms are predators; terrestrial forms are important soil aerators & nutrient recyclers; the gut is divided into specialized regions (pharynx, crop, gizzard, intestines); streamlined for burrowing; sensory structures are place in pits (referred to as epithelial sense organs); slime tube is secreted from clitellum that slides forward and collects egg & sperm(becomes a "cocoon" with eggs that hatch as young worms- direct development)

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Class Hirudinea

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blood-sucking & predatory leeches (generally freshwater); sometimes put into new class Clitellata; monoecious worms that cross-fertilize, have a clitellum that is only visible during reproduction, embryos develop inside cocoon where they develop directly into small worms; coelomic body cavity is filled with tissue, system of lacuna and sinuses surrounding the organs (no circulatory system); lack setae, have a fixed number of segments, no internal septa; primarily carnivorous ectoparasites with a posterior sucker that attaches to host, use anterior sucker to suck host's blood/body fluids; gut divided into specialized regions: pharynx (can include teeth or a protrusible proboscis), crop (no gizzard), intestine is short, salivary glands secrete aneasthetics, antibiotics, and anticoagulants of medical importance)

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atoke

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non-sexual form or part of polychaete

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epitoke

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sexual part or form of a polychaete

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Variety of polychaete lifestyles

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swim and/or crawl (long setae facilitate swimming); some polychaetes live in tubes or burrows (ex: "christmas tree" worms, sabellid "feather duster" tube worms, "peacock worms)

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Family Sabellidae

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filter-feeding tube worms; beating of the cilia on the pinnules of the radioles produces a current that flows upwards and out; particles are trapped on the pinnules and are driven by the cilia into a ciliated groove running the length of each radiole; the largest particles are rejected and fine material is carried into the mouth

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Family Siboglinidae

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"beardworms" are considered a clade (a group related by common ancestry) within class polychaeta based on DNA; some in marine sediments, others found in deep sea associated with geothermal/hydrothermal vents at 2600 meters depth; no digestive tract; two different feeding strategies (if small-tentacle and body wall directly absorb dissolved organic matter; in larger species at geothermal vents-trophosome is loaded with symbiotic mutualistic chemosynthetic bacteria that use sulfur oxidation, methane, and sulfide to to supply their annelid host with organic nutrients); dioecious: gonads are the only other organ system found in the turnk (eggs are brooded)

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Importance of leeches in medicine

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used for "blood-letting" years ago; used to prevent blood clots after severe injuries; leech saliva contains anaesthetic & anticoagulant ("hirudin") & helps prevent heart attacks and stroke, also a vasodilator & prostoglandin that reduces swelling; also harbor a bacterium that aids in the digestion of ingested blood & produces an antibiotic that kills other bacteria that may cause putrafaction at a wound site

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Phylum Echiura

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"spoonworms"; no sign of segmentation (evolutionarily "lost" it); constantly extended spoon shaped proboscis with a mouth at the end of the proboscis; complete gut; oval or sausage shaped trunk; proboscis uses mucus to trap food and cilia propel it along the food groove to the mouth; many are marine and live in U-shaped burrows; others live in rock and coral crevices; found at all ocean depths; dioecious, external fertilization, produce trochophore larva; closed circulatory system of contractile vessels (no heart); pairs of nephridia; ventral nerve cord & nerve ring (no brain)