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Nematodes Round Worms Remember: pg 2 Nematode Evolution pg 3 Nematode Notes There are many, many nematodes counting species as well as individuals 20,000 nematode species have been described but probably this is only a small fraction of the actual diversity The majority of nematodes are small predators or saprophytes (they eat decomposing organic matter) Nematode anatomy Long worms with often tapered ends Unsegmented Liquid filled body cavity (not lined by mesoderm: pseudocoel) Surrounded by rigid cuticle Simple organ and neuroanatomy Hypodermis organizes worm into quadrants cuticula Epicuticle: thin layer of lipids and proteins Cortex: made from cross-linked cuticilin proteins and collagen fibers Basal layers are formed by collagen fibers Basal layer is fibrous and closest to hypodermis cuticula Nematodes molt 4 times The old cuticle separates from the epidermis and the innermost layer is partially hydrolyzed A new cuticula is secreted by the hypodermis starting with the epicuticle The old cuticle is shed The hydrostatic skeleton and motility Nematodes have stiff motility Nematodes depend entirely on longitudinal muscles Nervous system The brain consists of a single ring around the pharynx A dorsal and a ventral nerve extend back through the length of the worm Labial and amphid nerves extend forward to enervate sensory organs (amphids and various papilla) Amphids are simple organs of mechano- and chemoreception. The sensory organelles are modified cilia Nematode digestive system, pumping against the pressure Nematodes have a complete (but simple) digestive system with a mouth at the very tip of the animal. The intestine is nonmuscular, and consists of a single layer tall columnar cells which carry microvilli and an underlying basal lamina Food has to be pumped against the pseudocoel pressure by the muscular esophagus Food is passed rapidly (minutes) Reproduction Most nematodes are dioecious (meaning they have two sexes, male and female), but hermaphrodites occur as well Males are smaller than females, and often have additional external features at the posterior end of the worm Development The development proceeds through 4 larval stages before reaching adulthood Molting is required at each step Many nematodes can develop a specialized resting or Dauer version of the L3 larval stage In parasitic nematodes this L3 larvae is often the infectious stage for the final host Nematodes impact human life Nematodes cause numerous human diseases Abundant pathogens in livestock and pets Pests of many crops Provide powerful genetic models to study the basis of development, aging and many diseases including cancer Wuchereria bancrofti Necator americanus Enterobius vermincularis Ascaris Trichinella spiralis Trichuris trichuria Nematoda Evolution Phylum Nematoda Class Secernentea Class Adenophorea Class Secernentea Ventrally coiled or derived therefrom Three esophageal glands, some with phasmids(nerves near anus); both free-living and parasitic forms. Wuchereria bancrofti Aka: Elephantiasis pg 392 Ingested by mesquitos and develop into their infective stage where they are transmitted into a new host Caused by adult filaria worms of Wuchereria bancrofti, which live in lymph passages and block the flow of lymph. Wuchereria bancrofti Necator americanus Hookworms pg 389 Named because their anterior end curves dorsally, suggesting a hook Large plates in their mouths cut into the intestinal mucosa of the host where they suck blood and pump it through their intestine, partially digesting it and absorbing the nutrients. Enterubius vermicularis Pinworms pg 391 Cause little disease Most common nematode parasite in the United States (30% of children and 16% of adults) Live in intestine and cecum Causes itching Ascaris Intestinal worm pg 388 Usually selected to study in Zoology Thus we know more about their structure, physiology and biochemistry than any other nematode A female may lay 200,000 eggs a day carried by the host feces. Eggs have an amazing tolerance to adverse conditions Ascaris Infection typically occurs when eggs are ingested with uncooked vegetables or when children put unsanitary things in their mouths. Ascauis megalocephala – found in the intestines of horses Ascaris lumbricoides – most common found in humans Class Adenophorea Amphids (nerves near mouth) generally well-developed, pocketlike 5 or more esophageal glands Phasmids (similar to Amphids but located on the posterior end) absent Excretory system lacking lateral canals Mostly free-living but includes some parasites Trichinella spiralis Trichinosis pg 390 Infection of all hosts occurs through larvae encysted in muscle tissue Females produce living young that enter the blood stream and are eventually found in almost any tissue of body space Trichinella Trichinella Muscle invasion stage begins in the second week after infection Fever and perorbital edema are followed by myalgia (muscle pain) and weakness Characteristic splinter hemorrhages can be found under finger nails Fever and chills can persist for weeks Headache is common and dizziness may develop Muscle swelling, aching and tenderness occurs often Deaths are rare and due to myocarditis (inflammation of the heart muscle), encephalitis and pneumonia (larvae in the diaphragm) Trichuris trichuria Whip Worm pg The third most common round worm of humans. Worldwide, with infections more frequent in areas with tropical weather and poor sanitation practices, and among children (800 million infected). Moderate to heavy infections cause abdominal pain, diarrhea, weight loss Adults can live for years so worm burden can build up Trichuris
