Biology 320 Invertebrate Zoology Fall 2005

1. Biology 320Invertebrate ZoologyFall 2005 Chapter 13 – Phylum Annelida Part Two

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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)

17. 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

19. Cocoons • 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. Coelom • 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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