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Biology 320 Invertebrate Zoology Fall 2005 Chapter 13 – Phylum Annelida Part Two Taxon Clitellata Two classes Class Oligochaeta Class Hirudinomorpha Posses a clitellum Several (6 or 7 in Lumbricus) anterior segments with a thick, glandular epidermis Conspicuous during reproduction Close proximity to gonopores Produces mucus for copulation, albumen for eggs, and cocoons Lack parapodia, prostomial and pygidial appendages Copulating hermaphrodites with direct development Class Oligochaeta Name means “few chaetae” 3500 spp. Earthworms are most familiar, but also small FW and marine varieties 200 marine spp., mostly interstitial Giant Australian earthworm (Megascolides australis) Up to 3 m long Body Form Only exceptions to generalized annelid are listed Four bundles of chaetae per segment Two ventral Two lateral Chaetae are structurally simple Genital chaetae are more complex One to 25 chaetae per bundle Two per bundle in most earthworms Protractor and retractor muscles attached to each chaeta Small in terrestrial varieties and longer in aquatics Coelom Earthworms posses coelomopores Connect coelomic cavities with exterior Posses a sphincter Pores are located in intersegmental furrows Exude coelomic fluid dorsally Keeps animal moist Deters predators “Squirter worm” (Didymogaster) can squirt fluid 30 cm high Locomotion Crawl or burrow using peristalsis Mucus and egested soil coat burrow walls Lumbricus terrestris covers burrow opening Larger worms can burrow deeper Chaetae are extended or retracted as needed to apply optimal traction Animal moves forward in steps 2 – 3 cm per step Approximately 30 cm per minute in some Can even crawl backwards Nutrition Scavengers of dead organic matter (such as leaves) and / or deposit feeders Swallow soil Takes 1 – 2.5 hr to process Castings are excreted Great for soil (aerates, mixes, and transfers nutrients) Aquatic varieties may be carnivores of amebas, ciliates, rotifers, etc. Some may parasitize FW snails Nervous System One large ventral nerve cord instead of two lateral cords Five giant axons Stimulation of either end of worm elicits an escape response (wriggling, or withdrawal into burrow) Subpharyngeal ganglion is motor control center Most lack eyes but have simple ocelli Some have rings of chemoreceptors (called tubercles) that project from cuticle Digestive System Pharyngeal bulb Eversible in aquatic spp. Muscular pump in terrestrial spp. Esophageal glands secrete mucus and enzymes Compartmentalized esophagus Crop for food storage Gizzard for grinding Lined with chitin-like cuticle Calciferous Glands Located in wall of esophagus Produce calcite crystals that are secreted into esophageal lumen and pass in feces Two functional hypothesis 1) Removal of excess CO2 Soil CO2 is high (relative to atmospheric levels), due to bacterial respiration Therefore there is a negative concentration gradient for diffusion of worm’s CO2 CO2 combines with calcium ions to form calcite 2) Removal of excess calcium Digestive System Cont… Intestine makes up posterior 3/4 of digestive system Anterior half secretes digestive enzymes Cellulase and chitinase (breaks down fungal cell walls) are released by mutualistic bacteria Posterior half is absorptive Typhlosole is a large dorsal fold in the intestine that increases its surface area Chlorogogen cells surround intestine Waste - laden cells are released into coelom Exit body via nephridiopores or coelomopores Circulation and Gas Exchange Often have hearts (described earlier) Five pairs in Lumbricus (segments 7-11) Gas exchange is carried out across body wall Large species have capillary loops in epidermis and hemoglobin dissolved in plasma Moist surface facilitates diffusion Oxygen levels in soil drop after heavy rains, forcing earthworms to the surface Excretion Ureotelic but excrete some ammonia Depends on environmental conditions Osmoregulation Urine is hyposmotic Often several types of nephridia Allows them to tolerate dry soils Those without special nephridia must burrow deeper during dry periods Encystment Can secrete tough mucus covering Summer cysts to avoid desiccation Winter cysts during periods of low temperature Undergo diapause Some migrate deeper into soil during dry or cold periods Up to three meters down in some 70% of body water can be lost Reproduction Clonal reproduction always occurs via transverse fission Sexual Monoecious (hermaphroditic) Reproductive organs located in a few anterior segments Paired ovaries release eggs which develop in ovisacs Paired testes release sperm which develop in seminal vesicles Genital segments each posses a pair of gonoducts (sperm ducts or oviducts), which open to the ventral surface Female genital segments have ventral openings that lead to seminal receptacles (store sperm prior to fertilization) Lumbricus breeds continually, but some have one yearly reproductive season Often reabsorb reproductive system and grow back later Copulation with mutual sperm transfer Ventral contact between oppositely oriented worms Genital chaetae and mucus produced by clitellum hold worms together Sperm swim in a ventral sperm groove from male gonopores to openings of partner’s seminal receptacles Indirect sperm transfer Entire process can take 2 – 3 hours Some worms have copulatory organ for direct sperm transfer Secreted a few days after copulation Clitellum secretes mucus tube Clitellum secretes chitinous material which becomes wall of cocoon Albumen from clitellum deposited into cocoon Tube slides forward as worm moves backwards Eggs and partner’s sperm (from seminal receptacles) deposited into cocoon External fertilization Cross fertilization Cocoon slides of head, and ends pinch off; mucus tube disintegrates Cocoons Terrestrial species deposit cocoons in soil Aquatic species deposit cocoons in debris or mud, or attach to vegetation Ovoid and yellow in color Contain 1 to 20 eggs 7.5 cm X 2 cm in Megascolides Development Direct development Therefore create eggs containing yolk or albumen Eight days to several months before juveniles emerge from cocoons Live several years Six years in captivity Reach sexual maturity at approximately 200 days Class Hirudinomorpha Leeches and closely related worms 500 spp Marine, mostly freshwater, a few terrestrial species (restricted to moist environments) Lack chaetae Have a fixed number of segments (typically 33) All have a posterior sucker for adhering to prey or substratum Most have an anterior sucker as well Blood-sucking ectoparasites or carnivores 1 cm to 30 cm long (Haementeria, giant Amazonian leech) Black, brown, olive, or red in color May have striped / spotted patterns Typically inhabit stagnant or slow moving freshwater Estivate in mud during periods of drought Can lose 90% of body water Often extremely abundant Body Form Dorsoventrally flattened Tapered at anterior end Suckers Anterior (if present) is smaller and surrounds mouth Posterior is disc-shaped; anus located directly anterior Annulations that don’t accurately depict segments Number of annulations per segment varies Clitellum spans segments 9-11, but is only conspicuous during reproduction Body Wall Connective tissue is much thicker Makes up a larger proportion of the animal Slightly different musculature Dorsoventral muscles For flattening Helical muscles For twisting Larger connective tissue compartment means a reduction of the coelom Lack septa, so therefore lack bilateral coelomic cavities Also lack mesenteries Continuous coelom which serves as a hemal system Two large lateral coelomic vessels Dorsal and ventral coelomic vessels Lined with mesothelium (chlorogogen cells) Muscle contractions propel fluid Body surface conducts gas exchange Coelom Locomotion Not equipped for burrowing due to: Reduction of coelom Loss of septa and chaetae Many inchworm by anchoring with anterior and posterior suckers Many swim First contract dorsoventral muscles to increase surface area Then undulate Nervous System Similar to other annelids Fusions of ganglia in the sucker regions Posses ocelli Posses sensory papillae Projecting discs consisting of many sensory cells Typically found dorsally, in rows, on one annulation of a particular segment Sense organs mainly used for prey detection, and respond to: Moving shadows and water-pressure vibrations in fish leeches Chemicals from body fluids such as oils, sweat, and blood Waves Temperature differences Excretory System 10 – 17 pairs of metanephridia One pair per segment in middle 1/3 of animal Nephrostomes project into coelomic vessels Nephridial tubules are embedded in connective tissue Posses a bladder that fills before urine is released through nephridiopore Important for osmoregulation Digestive System Pharynx types Protrusible pharynx that is forced into prey / host tissue Non-protrusible sucking pharynx May or may not have jaws Enzymes usually facilitate penetration in those lacking jaws Salivary glands empty secretions into pharynx Hirudin – anticoagulant Anesthetic Vasodilators Often 1 – 11 pairs of lateral intestinal ceca Nutrition Three fourths are blood suckers Usually not host specific Usually parasitize a group of organisms (i.e. crayfish, fish, reptiles, birds, mammals, etc.) Some are carnivores of small invertebrates Water from plasma is excreted via nephridiopores Digestion is very slow Gut produces hardly any enzymes Symbiotic bacteria may produce digestive enzymes and vitamins May take 200 days to digest a blood meal Rarely feed One blood meal may increase animals weight by 10X May only need to feed twice a year in order to grow Some can fast for 1.5 years Medical Applications Historically used in Europe for bloodletting Supposedly, George Washington died two days after a bloodletting for a soar throat Hirudo medicinalis is still used today for restoring circulation and reducing swelling after: Skin grafts Digit / appendage reattachment Natural antibiotic properties Symbiotic bacteria produce antibiotics to reduce competition with other bacteria Reproduction No sexual reproduction, including regeneration Hermaphroditic, but not simultaneous Protandric Male portion of reproductive system develops first Spend first part of existence as males, then later reproduce as females Copulation and fertilization is always internal Most have copulatory organ for direct sperm transfer Others hypodermically impregnate using a spermatophore Pressure and cytolytic chemicals rupture body wall of mate Sperm are released and migrate toward eggs Eggs are laid after copulation Time varies Clitellum secretes a cocoon and albumen The cocoons of fish leeches are attached to fish hosts Some brood eggs by attaching cocoon to substratum and ventilate eggs by fanning flattened ventral surface Some attach cocoon to ventral surface