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Download File - Invertebrate Zoology
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Phylum Nematoda Roundworms 20,000 described species – May be the largest animal phylum Found worldwide and live in all habitat types – – – – Interstitially Inside of animals and plants 53°C hot springs Epiphytic bromeliads Very abundant – One cubic meter of mud off of the coast of Holland contains over 4,000,000 nematodes Ecologically important – Link between decomposers and higher trophic levels – Some are excellent decomposers and nutrient cyclers 90,000 nematodes in one rotting apple Caenorhabditis elegans is an Eutelic important model organism for genetics / development studies – C. elegans has 1031 cells Body Form Vermiform Tapered at both ends – Important adaptation for living interstitially Microscopic to 50 cm – One mm is typical Mouth at anterior end, with three to six lips that bear sensilla Caudal gland on posterior that is similar to a duo-gland Lack cilia Body Wall Cuticle – Not chitinous – Must be molted Epidermis – – – – Secretes cuticle Stores nutrients Endoparasites may absorb nutrients with Has four longitudinal extensions called epidermal cords Molted four times while growing Molting ceases upon reaching adulthood Ecdysone controls molting Longitudinal nerve cords housed here Musculature – Four bands of longitudinal fibers separated by epidermal cords – No circular muscles Locomotion Use sinusoidal undulations of dorsoventral plane to move forward – Alternate contractions of dorsal and ventral longitudinal muscles Efficient movement requires a substratum to act against – Removing worms from natural substratum hinders locomotion Many swim or are capable of swimming Some crawl like earthworms or inchworms Organ Systems Hemal system – Large nematodes have a hemocoel (pseudocoel) Fluid filled and fluid may contain hemoglobin Functions as hydrostat – Small varieties are acoelomate No respiratory system Nervous system – Collar-like brain surrounds pharynx – Dorsal, ventral, and lateral longitudinal nerve cords in epidermal cords – Sensory organs Papillae – low projections of cuticle on lips and head Tactile setae Ocelli – one on each lateral aspect, near pharynx Nutrition Will eat just about anything – Carnivorous, herbivorous, or omnivorous – Will eat bacteria, protists, fungi, other nematodes, and plant cells Some are deposit or detritus feeders – Actually feed on bacteria associated with these types of organic matter Some fungi actually prey on nematodes – Trap them with hyphal threads Digestive System Similar to that of gastrotrichs Cuticle of foregut contains teeth, ridges, rods, or plates – Varies according to feeding habits Example: Mononchus uses teeth to latch onto other nematodes Body contents are pumped out Will eat over 1000 nematodes over its 14 week life span – Diagnostic tool for taxonomists Herbivores have a stylet for piercing plant cells Some parasites use body surface to absorb nutrients Reproduction Some hermaphroditic Some parthenogenic Internal fertilization with copulation Sexually dimorphic – Males smaller than female – Male has copulatory spicules (resemble curved blades) that are used to hold female gonopore open Females produce pheromones to attract males Sperm are aflagellate and amoeboid Eggs have characteristic shapes, and experts can use these shapes to diagnose infections Some free-living types, such as Turbatrix, are viviparous (i.e., give birth to live young) Hermaphroditic varieties typically self-fertilize and rarely crossfertilize – Little genetic recombination Egg numbers vary greatly between species – 50 in some marine species – 200,000 per day in the parasite Ascaris Eutelic – Around 1000 cells – Number of cells in individual organs is also constant – Cells grow in size, not in number Direct development – Egg, three juvenile instars, and adult – Four molts (first two of which may occur in egg before hatching) – Molting ceases upon reaching adulthood Parasitism Can be ectoparasites or endoparasites Can have one host or multiple hosts Can infect plants, animals, or both Juveniles, adults or both can be infective Ascaroid nematodes – Ascaris, hookworms, pinworms, and Trichinella – Infect one host: humans, cats, dogs, pigs, cows, etc. – Infection occurs when eggs or juveniles are ingested Eggs hatch in intestine Juveniles penetrate intestinal wall Juveniles enter hemal system (kidney system) Juveniles break through alveoli in lungs They migrate up respiratory tree Are coughed up in sputum If sputum is swallowed, end up in intestine Ascaris – Human intestinal worm: Ascaris lumbricoides – Can reach 50 cm in length – Widely distributed, but is found in SW US – Eggs found in soil and are resistant to harsh environmental conditions – Children are susceptible to infection because they put things in their mouths – Feeds on fluid gut contents of host – Can cause malnutrition and death in the event of intestinal blockage – Secrete substances which inhibit host digestive enzymes from harming them Hookworms – Necator americanus – Juveniles enter through skin of foot – Follow typical ascaroid path – Attach to intestinal lining with hooks – Feed on host’s blood – Infections of more than 25 worms can lead to serious blood loss and tissue damage – Common in tropics – 380 million infected worldwide Pinworms – Enterobius vermicularis – Typically infect humans – Worldwide distribution – Adults live in intestine – At night, female crawls out anus and deposits eggs – Scratching traps eggs under fingernails Trichinella spiralis – Infects mammals with a condition known as trichinosis – Juveniles are carried to skeletal muscles by blood Form calcified cysts in muscle (bladder worm) – Transmitted when undercooked flesh is eaten Example: humans eating undercooked pork – Severe infection causes pain and stiffness The following groups of parasitic nematodes have two hosts (intermediate and definitive) – Filarioids – Dracunculoids Filarioids – Elephantiasis, heart, and eye worms – Thin, threadlike worms that inhabit the lymphatic system or other tissues – Intermediate host is typically a blood sucking insect – Definitive host is usually a bird or mammal Wuchereria bancrofti (Elephantiasis) – Found mainly in Africa and Asia – Intermediate host is a mosquito, definitive host is a human – Threadlike adults live in lymph glands Blockage of lymph vessels causes edema – Long term blockage causes a condition known as elephantiasis Enlargement of appendages, breasts, scrotum, etc… – Life cycle Eggs hatch into microfilariae that migrate to surface blood vessels at night (when mosquitoes are biting) Microfilariae migrate from mosquito gut lumen to proboscis Injected into definitive host Wuchereria bancrofti (Elephantiasis) Loa Loa – African eye worm – Sometimes crosses cornea Loa Loa – African eye worm – Sometimes crosses cornea Loa Loa Dracunculus medinensis – A dracunculoid – Guinea worm or Dragon worm – Intermediate host is a FW copepod and definitive host is human – Adult female lives below skin of human and produces an ulcer Juveniles released in water Ingested by a copepod Humans ingest copepod by drinking contaminated water – Worms removed surgically or by winding on a stick Caduceus (symbol of medical profession) is really a nematode curled around a stick Dracunculus medinensis Dracunculus medinensis Phylum Gastrotricha 500 species Found in all three habitat types – Marine and FW - interstitially or on plants – Terrestrially - in water film covering soil particles Microscopic to 4mm long Eutelic – Genetically predetermined and constant number of cells – Cell size increases, but not cell number Body Form Bowling-pin-shaped – Name means “hairy belly” Locomotory cilia on ventral surface Adhesive tubes located near head and/or on lateral body – Adhesive organs at posterior – Similar to duo-glands of turbellarians Body Wall Cuticle – Not chitinous – Not molted Epidermis Musculature – Circular – Longitudinal Organ Systems Acoelomates – no body cavity No respiratory system Nervous system – Brain – Pair of ventrolateral nerve cords – Sensory bristles found on body surface Excretory system – One to several pairs of protonephridia Produces ammonia as waste product Movement Ciliary gliding for forward movement Rapid rearward withdrawal as an escape response Can use a combination of adhesion and muscular contractions to: – “Inchworm” along – Somersault Nutrition Gut lacks stomach Large muscular pharynx – Y- or T-shaped in some – Possess pharyngeal pores that open to surface and release excess ingested water Feed on small organic particles (live or dead) – Bacteria and protozoans Reproduction Hermaphroditic with indirect sperm transfer Copulatory organ is loaded with sperm from male gonopore Spermatophore transferred to second individual via posterior female gonopore Internal fertilization the occurs Fertilized eggs are released by rupturing body wall Some species are parthenogenic Can produce two types of eggs – Summer – Winter (resting) Direct development Sexual maturity is reached in about three days Life span is approximately 40 days
