ch. 15 nematoda (web version)

8/6/2019 Ch. 15 Nematoda (Web Version)

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Chapter 15:

Phylum Nematoda:The Roundworms


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Nematode = Thread?

Roundworms were given the namenematodes because they resemble a thread.

In Greek, nematosactually means thread,hence why they are called nematodes.

About 12,000 species have been named, butestimates say that if we knew about all the

different species, that number would becloser to 500,000.


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What is a Roundworm?

Unlike the platyhelminthes, nematodes havea digestive tract with two openings. Thisbody plan is often called a tube-within-a-

tube. The outer tube is the body wall andthe inner tube is the digestive tract.

This arrangement makes digestion inroundworms very different from that inflatworms because food moves in onedirection through the digestive tract.


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Form and Function in Roundworms

Roundworms have specialized tissues andorgan systems that carry out essential bodyfunction.

In general, the body systems of free-livingroundworms tend to be more complex thanthose of parasitic forms.

Distinguishing characteristics of this phylumare their cylindrical shape, flexible nonlivingcuticle, lack of motile cilia or flagella, lack ofprotonephridia, and the muscles of their

body wall run only longitudinally.


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Body Covering

The outer body covering is a relatively thick,noncellular cuticle secreted by the underlyingepidermis, or hypodermis.

The several layers of the cuticle are primarilyofcollagen, a structural protein alsoabundant in vertebrate connective tissue.


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Feeding

Many free-living roundworms are carnivoresthat use grasping mouthparts and spines tocatch and eat other small animals.

Some soil-dwelling and aquatic forms eatalgae, fungi, or pieces of decaying matter.

Other nematodes digest the bacteria and

fungi that break down dead animals andplants.


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Respiration, Circulation, and Excretion

Like flatworms, roundworms exchange gases(respire) and excrete metabolic wastes likeurea and ammonia through their body walls.

They have no internal transport system.

Diffusion is the main process through whichnutrients and wastes are carried through

their bodies.


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Response

Nematodes have simple nervous systems,consisting of several ganglia.

Several nerves extend from ganglia in the

head and run the length of the body. Thesenerves transmit sensory information andcontrol movement.

Roundworms have several types of senseorgans.


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Response

A ring of nerve tissue and ganglia are foundat the anterior end of their bodies.

They have a pair ofamphids that are

somewhat more complex sense organs thatopen around their heads.

They also have a pair ofphasmids that are

similar in structure as amphids, but openaround the posterior end of the body.


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Movement

The muscles of nematodes extend the lengthof their bodies. Together with the fluid inthe pseudocoelom, these muscles function as

a hydrostatic skeleton. A hydrostatic skeleton is the use of

coelom fluid to maintain the shape of theanimal and allows for locomotion.

Aquatic roundworms contract these musclesto move like snakes through the water.

Soil-dwelling roundworms push their waythrough the soil by thrashing around.


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Muscular System

Body wall muscles of nematodes are veryunusual. They have no circular muscles.

Each muscle cell has a contractile fibrillar

portion (composed of fibers), and anoncontractile sarcoplasmic (clear,semifluid cytoplasm between the fibers in themuscle fibers) portion.


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Reproduction

Roundworms reproduce sexually, and mostspecies of roundworms are dioecious.

They reproduce using internal fertilization:

the male usually deposits sperm inside thefemales reproductive tract.

Parasitic nematodes often have complex life

cycles that involve two or three differenthosts or several organs within a host.


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Classes of Nematoda

There are two main classes of nematodes:Rhabditea and Enoplea. Class Rhabditea Amphids are ventrally coiled or

derived therefrom; three esophageal glands;some with phasmids; there are both free-livingand parasitic forms. Examples: Caenorhabditis,

Ascaris, Enterobius, Necator, Wucheria

Class Enoplea Amphids usually well-developed,

pocket-like; five or more esophageal glands;phasmids absent; excretory system lacking lateralcanals; formed of single, ventral, glandular cells,or entirely absent; mostly free living, but includessome parasites. Examples: Dioctophyme,

Trichinella, Trichuris.


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Caenorhabditis elegans

In 1963 Syndey Brenner started studyingCaenorhabditis elegans, a free-livingnematode. It has turned out to be one of

the most important experimental models inbiology.

It is easy to grow in the laboratory, and wehave been able to map the genome and thewiring diagram of the nervous system ofthis roundworm.


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Ascaris lumbricoides

Ascaris lumbricoides occurs in up to 64% ofpeople in some areas of the southeasternU.S. More than 1.2 billion are affected

worldwide. A. megalocephala is found in the intestine of

horses. A. suum is found in pig intestines.


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A femaleAscaris can lay 200,000 eggs perday, passing out through the hosts feces.In good soil, embryonation is complete in

two weeks. Viable eggs remain after signs of fecal

matter have disappeared. Eggs can survivelong periods in the soil.

Barefoot? I think not.


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When a host swallows the eggs, juvenileshatch and burrow through the intestinal wall.

The juveniles then are carried through the

heart to the lungs. When at the lungs, theybreak into the alveoli and are carried up tothe trachea.

Juveniles are coughed up and swallowed,

then mature in the intestine two monthsafter they were swallowed.

They feed on intestinal contents and mayblock or perforate the intestines.


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Hookworms

Hookworms are so named because theanterior (head) end curves dorsally,resembling a hook. They are dioecious.

Necator americanus is most commonspecies.

They have large plates in their mouths thatcut into the intestines so that they can suckon the hosts blood.


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Hookworms

Hookworms pump more blood than they candigest. A heavy infection can cause anemia.

Eggs pass in feces and juveniles hatch in soil

where they can live off of bacteria.

If human skin comes in contact with the soil,infective juveniles burrow through the skin toblood.

Their life cycle is similar to that ofAscaris.


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Trichina Worm

Trichinella spiralis is tiny, but causes apotentially lethal trichinosis.

Adult worms burrow into the intestines and

females directly produce living young. Thejuveniles penetrate blood vessels andcirculate throughout the body to all tissuesand spaces.


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Trichina Worm

Juveniles penetrate skeletal muscle cells,change its gene expression so that itbecomes a nurse cell to the parasite.

When meat containing live juveniles is eaten,worms are liberated and mature in theintestine.


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Trichina Worm

They infect humans, hogs, rats, cats, anddogs. Hogs can become infected eatinguncooked scraps of infected meat or rats.

There are four other sibling species withvariable distribution, freezing resistance, etc.

Heavy infections can cause death but lighterinfections are more common. About 2.4% ofthe U.S. population is infected, mostlylightly.


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Pinworms

Pinworms are the most common wormparasite in the U.S., but causes little disease.

It is estimated that 30% of children and 16%

of adults in the U.S. have them.

Adults live in the large intestine and cecum.


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Pinworms

Females, about 12 mm in length, migrate tothe anal region at night and lay eggs,causing itching. Scratching the anal region

contaminates hands and bedclothes.


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Pinworms

Eggs develop rapidly and become infectivewithin six hours at body temperature.

When swallowed, these eggs hatch in the

anterior end of the small intestine (theduodenum) and mature in the largeintestine.

Members of this order have haploid (one setof chromosomes) males from unfertilizedeggs and diploid females from fertilizedeggs. This is known as haplodiploidy.


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Filarial Worms

Mosquitoes ingest the microfilariae whenthey feed. The worms develop to theinfective stage while inside the mosquito and

move into the mosquito bite wound when itfeeds.


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Filarial Worm Diseases

Filarial worms causethree main diseases intheir definitive hosts:

Elephantiasis River blindness

Dog heartworm


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Elephantiasis

Elephantiasis symptoms areapparent after long andrepeated exposure to

filarial worms. It is marked by excessive

growth of connective tissueand enormous swelling of

affected parts.


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Onchocerciasis: River blindness

River blindness iscaused by a filarialworm that is carried by

black flies. It infects more than 30

million people in partsof Africa, Arabia,

Central America, SouthAmerica, and virtuallyall other tropical areas.


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DogHeartworm

The most common filarial worm in the U.S. isthe dog heartworm, Dirofilaria immitis.

It is carried by mosquitoes and can infect

canines, cats, ferrets, sea lions andsometimes humans.

Prevalence in dogs along the Atlantic andGulf coast states as well as the Midwest is

about 45%. It occurs in other states, butisnt as common.

It causes a very serious disease amongdogs, and owners should provide dogs withheartworm pills during mosquito season.

ch. 15 nematoda (web version)

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