chapter 36: flatworms, roundworms, and rotifers

Chapter 36: Flatworms, Roundworms, and Rotifers

Objectives:• State the distinguishing characteristics of flatworms• Describe the anatomy of a planarian• Compare and contrast free-living and parasitic flatworms• Diagram the life cycle of a fluke• Describe the life cycle of a tapeworm• Describe the body plan of a pseudocoelomate• Explain the relationship between humans and three types of parasitic

roundworms• Describe the anatomy of a rotifer

36-1: Platyhelminthes• Members of the phylum Platyhelminthes are

called flatworms. • Their bodies develop from three germ layers and

are more complex than those of sponges, cnidarians, and ctenophores.

• Flatworms have Bilateral symmetry, with dorsal and ventral surfaces, right and left sides, and anterior and posterior ends.

What Is a Flatworm?• Flatworms are soft, flattened worms

that have tissues and internal organ systems.

• They are the simplest animals to have three embryonic germ layers, bilateral symmetry, and cephalization.

What Is a Flatworm?•Flatworms are acoelomates, which means they have no coelom. •A coelom is a fluid-filled body cavity that is lined with tissue derived from mesoderm. •The digestive cavity is the only body cavity in a flatworm.•Flatworms have bilateral symmetry.

What Is a Flatworm?•Three germ layers of a flatworm

Form and Function in Flatworms

• Flatworms are thin and most of their cells are close to the external environment.

• All flatworms rely on diffusion for respiration, excretion, and circulation.


•Free-living flatworms have organ systems for digestion, excretion, response, and reproduction.•Parasitic species are typically simpler in structure than free-living flatworms.

Form and Function in Flatworms•Feeding – Flatworms have a digestive cavity with a

single opening through which both food and wastes pass.

– Near the mouth is a muscular tube called a pharynx.

– Flatworms extend the pharynx out of the mouth. The pharynx then pumps food into the digestive cavity.


•Most parasitic worms do not need a complex digestive system.•They obtain nutrients from foods that have already been digested by their host.

Form and Function in Flatworms•Respiration, Circulation, and Excretion – Flatworms do not need a circulatory system to

transport materials.– Flatworms rely on diffusion to

• transport oxygen and nutrients to their internal tissues, and

• to remove carbon dioxide and other wastes from their bodies.


•Flatworms have no gills or respiratory organs, heart, blood vessels, or blood.•Some flatworms have flame cells which are specialized cells that remove excess water from the body.•Flame cells may filter and remove metabolic wastes.


•Response – In free-living flatworms, a head encloses

ganglia, or groups of nerve cells, that control the nervous system.

– Two long nerve cords run from the ganglia along both sides of the body.


•Many free-living flatworms have eyespots.•Eyespots are groups of cells that can detect changes in light. •Most flatworms have specialized cells that detect external stimuli.•The nervous systems of free-living flatworms allow them to gather information from their environment.


•Digestive Structures of a Planarian

Testes

Ovary

Ganglia

Nerve cords

Excretory system

Flame cell

Excretory tubule

•Excretory, Nervous, and Reproductive Structures of a Planarian

Form and Function in Flatworms•Movement – Free-living flatworms move in two ways. – Cilia on their epidermal cells help them glide

through the water and over the bottom of a stream or pond.

– Muscle cells controlled by the nervous system allow them to twist and turn.

Form and Function in Flatworms•Reproduction – Most free-living flatworms are hermaphrodites

that reproduce sexually. – A hermaphrodite is an individual that has both

male and female reproductive organs. – Two worms join in a pair and deliver sperm to

each other. – The eggs are laid in clusters and hatch within a

few weeks.

Form and Function in Flatworms• Asexual reproduction takes place by fission,

in which an organism splits in two.• Each half grows new parts to become a

complete organism. • Parasitic flatworms often have complex life

cycles that involve both sexual and asexual reproduction.

Classes of Flatworms

•What are the characteristics of the three Classes of flatworms?

Classes of Flatworms

• The three main groups of flatworms are– turbellarians– Trematoda- flukes– Cestoda- tapeworms

• Most turbellarians are free-living. • Most other flatworm species are parasites.

Classes of Flatworms•Class Turbellarians– Turbellarians are free-living flatworms. Most

live in marine or fresh water.– Most species live in the sand or mud under

stones and shells.

Classes of Flatworms•Class Trematoda- Flukes– Flukes are parasitic flatworms. Most flukes infect

the internal organs of their host. Usually don’t kill the host.

– Host are typically Animals and humans– No special sense organs– Mostly aquatic– Most less than 1cm long– Schistosoma- Genus of flukes that cause

Schistosomiasis.

Form and Function in Flatworms• Flukes can infect the blood or organs of the

host.• Some flukes are external parasites.• In the typical life cycle of parasitic flukes, the

fluke lives in multiple hosts.

Form and Function in Flatworms•Life Cycle of a Blood Fluke

Form and Function in Flatworms•A blood fluke’s primary host is a human.•Blood flukes infect humans by burrowing through the skin.

Human intestine

Tailed larva

Form and Function in Flatworms•Once inside the human, they are carried to the blood vessels of the intestines. •In the intestines the flukes mature and reproduce. •Embryos are released and are passed out of the body with feces.

Adult fluke

Embryo

Form and Function in Flatworms•If the embryos reach water, they develop into swimming larvae that infect a snail (the intermediate host).•An intermediate host is an organism in which a parasite reproduces asexually.

Life Cycle of a Blood Fluke

Embryo

Ciliated larva

Form and Function in Flatworms•Larvae that result from asexual reproduction are released from the snail into the water to begin the cycle again.

Life Cycle of a Blood Fluke

Form and Function in Flatworms•Class Cestoda- Tapeworms – Tapeworms are long, flat, parasitic worms that

are adapted to life inside the intestines of their hosts.

– No organs for locomotion, senses, or digestion. They absorb from the hosts digestive system.

– Can be up to 40 feet long.


• Tapeworms have no digestive tract and absorb digested food directly through their body walls.

• The head of an adult tapeworm, called a scolex, is a structure that can contain suckers or hooks.

• The tapeworm uses its scolex to attach to the intestinal wall of its host.

•Structures of a Tapeworm


Form and Function in Flatworms• Proglottids are the segments that make up

most of the worm's body. • Mature proglottids contain both male and

female reproductive organs. • Sperm produced by the testes (male

reproductive organs), can fertilize eggs of other tapeworms or of the same individual.


• After the eggs are fertilized, the proglottids break off and burst to release the zygotes.

• The zygotes are passed out of the host in feces.

• The eggs ingested by an intermediate host hatch and grow into larvae.

• Larvae burrow into the intermediate host’s muscle tissue.

Form and Function in Flatworms• Larvae form a dormant protective stage called

a cyst. • If a human eats incompletely cooked meat

containing these cysts, the larvae become active and grow into adult worms within the human’s intestines, beginning the cycle again.

36-2: Nematoda and Rotifera• Members of the Phyla Nematoda and Rotifera

have bilateral symmetry and contain a fluid-filled space.

• This space holds the internal organs and serves as a storage area for eggs and sperm.

• It also supports the body and provides a structure against which the muscles can contract.

Phylum Nematoda• Nematoda is made up of roundworms, worms

with long slender bodies that taper at both ends.• Roundworms are Pseudocoelomates• Pseudocoelomates is a hollow fluid filled cavity

that has mesoderm lining the outside and endoderm on the inside. (coelomates have mesoderm lining the entire cavity)

• Roundworms are un-segmented worms that have pseudocoeloms and digestive systems with two openings—a mouth and an anus.

What Is a Roundworm?

•This cavity is partially lined with tissue derived from the mesoderm and is called a pseudocoelom, meaning, “false coelom.”


• Roundworms have a digestive tract with two openings.

• Food moves in one direction through the digestive tract of roundworms.

• Any food that is not digested leaves the body through the anus.


Form and Function in Roundworms

• Roundworms have specialized tissues and organ systems that carry out essential physiological functions.


•Feeding– Many free-living roundworms use grasping

mouthparts and spines to catch and eat other small animals.


•Respiration, Circulation, and Excretion– Roundworms exchange gases and excrete

metabolic waste through their body walls.– They depend on diffusion to carry nutrients and

waste through their bodies.


•Response– Roundworms have simple nervous systems,

consisting of several ganglia. – Several nerves extend from ganglia in the

head and run the length of the body. – These nerves transmit sensory information

and control movement.


•Movement– Fluid in the pseudocoelom and muscles

extending the length of their bodies function as a hydrostatic skeleton.

– Aquatic roundworms contract muscles to move like snakes through the water.

– Soil-dwelling roundworms push their way through the soil by thrashing around.


•Reproduction – Roundworms reproduce sexually.– Most species have separate sexes.– Roundworms reproduce using internal

fertilization.– Parasitic roundworms often have life cycles

that involve two or three different hosts or several organs within a single host.

•What roundworms cause human disease?

Roundworms and Human Disease

•Parasitic roundworms include trichinosis-causing worms, filarial worms, ascarid worms, and hookworms.



•Trichinosis-Causing Worms – Adult Trichinella worms live and mate in the

intestines of their hosts.– Female worms carrying fertilized eggs burrow

into the intestinal wall and release larvae. – Larvae travel through the bloodstream and

burrow into organs and tissues.

• The larvae form cysts and become inactive in the host’s muscle tissue.

• Trichinella completes its life cycle only when another animal eats muscle tissue containing these cysts.

• Humans can get trichinosis by eating raw or incompletely cooked pork.



•Filarial Worms – Filarial worms are threadlike worms that live in

the blood and lymph vessels of birds and mammals.

– They are transmitted by biting insects.– Large numbers of filarial worms may block the

passage of fluids within lymph vessels and cause swelling.


•Ascarid Worms – Ascaris lumbricoides is a serious parasite of

humans and many other vertebrate animals. – It absorbs digested food from the host’s small

intestine. – Ascaris lumbricoides is commonly spread by

eating foods that are not washed properly.

Ascaris Life Cycle

Human ingests food or water containing Ascaris eggs.


1

Ascaris Life Cycle

The eggs travel to the small intestine and develop into larvae.


2

Roundworms and Human DiseaseAscaris Life Cycle

Larvae enter blood vessels and are carried to the lungs

3

Ascaris Life Cycle

Larvae are coughed up and swallowed. They then travel to the small intestine where they develop to maturity


4

Ascaris Life Cycle

Eggs are released and leave the host in faces.


5


•Hookworms – Hookworm eggs hatch and develop in the soil. – They use sharp tooth like plates and hooks to

burrow into the skin and enter the bloodstream.

• Hookworms travel through the blood of their host to the lungs and down to the intestines.

• There, they suck the host’s blood, causing weakness and poor growth.


Phylum Rotifera

• Most Rotifers are transparent, free-living animals that live in fresh water.

• When they are in dry conditions the look like grains of sand and then will go back to their normal state when wet.

• Have cilia near the mouth. Looks like a pair of rotating wheels. They sweep food- algae, bacteria, and protozoans- into their digestive tracks.

Rotifers

chapter 36: flatworms, roundworms, and rotifers

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