Flatworms, Roundworms, and RotifersChapter 34

Table of Contents

Section 1 Platyhelminthes

Section 2 Nematoda and Rotifera

Section 1 PlatyhelminthesChapter 34

Objectives

• Summarize the distinguishing characteristics of flatworms.

• Describe the anatomy of a planarian.

• Compare free-living and parasitic flatworms.

• Diagram the life cycle of a fluke.

• Describe the life cycle of a tapeworm.

Section 1 PlatyhelminthesChapter 34

Structure and Function of Flatworms

• The phylum Platyhelminthes includes organisms called flatworms.

• They are more complex than sponges but are the simplest animals with bilateral symmetry.

• Their bodies develop from three germ layers:– ectoderm– mesoderm– endoderm

• They are acoelomates with dorsoventrally flattened bodies.• They exhibit cephalization.• The classification of Platyhelminthes has undergone many

recent changes.

Chapter 34

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Visual Concept

Characteristics of Flatworms

Section 1 Platyhelminthes

Section 1 PlatyhelminthesChapter 34

Class Turbellaria

• The majority of species in the class Turbellaria live in the ocean.

• The most familiar turbellarians are the freshwater planarians of the genus Dugesia.

• Planarians have a spade-shaped anterior end and a tapered posterior end.

Section 1 PlatyhelminthesChapter 34

Class Turbellaria, continued

Digestion and Excretion in Planarians

• Planarians feed on decaying plant or animal matter and smaller organisms.

• Food is ingested through the pharynx.• Planarians eliminate excess water through a network

of excretory tubules. – Each tubule is connected to several flame cells.– The water is transported through the tubules and

excreted from pores on the body surface.

Section 1 PlatyhelminthesChapter 34

Class Turbellaria, continued

Neural Control in Planarians

• The planarian nervous system is more complex than the nerve net of cnidarians.

• The cerebral ganglia serve as a simple brain. • A planarian’s nervous system gives it the ability to

learn. • Planarians sense light with eyespots. • Other sensory cells respond to touch, water currents,

and chemicals in the environment.

Section 1 PlatyhelminthesChapter 34

Class Turbellaria, continued

Reproduction in Planarians

• Planarians are hermaphrodites that can reproduce sexually or asexually.

• Their eggs are laid in capsules. • During asexual reproduction, their body undergoes

fission and the two halves regenerate missing parts.

Chapter 34

Planarian

Section 1 Platyhelminthes

Chapter 34

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Visual Concept

Anatomy of a Planarian

Section 1 Platyhelminthes

Chapter 34

Exploration of a Flatworm

Section 1 Platyhelminthes

Chapter 34

Development of Flatworm Embryo

Section 1 Platyhelminthes

Section 1 PlatyhelminthesChapter 34

Classes Trematoda and Monogenea

• The classes Trematoda and Monogenea consist of parasitic flukes.

• Some are endoparasites; others are ectoparasites.

Structure of Flukes• A fluke clings to the tissues of its host by an anterior sucker and

a ventral sucker.• A fluke’s nervous system is similar to a planarian’s, but simpler. • The external surface of a fluke is covered by a protective layer

called the tegument.

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Tegument

Section 1 Platyhelminthes

Section 1 PlatyhelminthesChapter 34

Classes Trematoda and Monogenea, continued

Reproduction and Life Cycle of Flukes• Most flukes have highly developed reproductive

systems and are hermaphroditic.• Fertilized eggs are stored in a fluke’s uterus until they

are ready to be released. • Flukes have complicated life cycles that involve more

than one host species.• For example, the trematode blood flukes of the genus

Schistosoma use humans as a primary hosts and snails as intermediate hosts. They can cause schistosomiasis.

Chapter 34

Life Cycle of Schistosoma

Section 1 Platyhelminthes

Chapter 34

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Visual Concept

Life Cycle of Flukes

Section 1 Platyhelminthes

Section 1 PlatyhelminthesChapter 34

Class Cestoda

• About 5,000 species of tapeworms make up the class Cestoda.

• Tapeworms can live in the intestines of almost all vertebrates.

Structure of Tapeworms• Tapeworms are surrounded by a tegument.• They attach to the host with a scolex.• The body is a series of many sections called

proglottids.• Tapeworms have no light-sensing organs, no mouth,

no gastrovascular cavity, and no digestive organs.

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Visual Concept

Anatomy of a Tapeworm

Section 1 Platyhelminthes

Section 1 PlatyhelminthesChapter 34

Class Cestoda, continued

Reproduction and Life Cycle of Tapeworms

• Nearly all tapeworms are hermaphrodites. Each proglottid contains male and female reproductive organs and little else.

• New proglottids are added to the front of the tapeworm. Older proglottids grow, mature, and begin producing eggs.

• Eggs in one proglottid are usually fertilized by sperm from a different proglottid, possibly a different individual.

• An example is the beef tapeworm, Taenia saginatus. Its primary host is a human and its intermediate host is a cow. Its larvae form cysts in the muscle tissue of the cow.

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Life Cycle of Beef Tapeworm

Section 1 Platyhelminthes

Chapter 34

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Visual Concept

Life Cycle of Tapeworms

Section 1 Platyhelminthes

Section 2 Nematoda and RotiferaChapter 34

Objectives

• Describe the body plan of a nematode.

• Outline the relationship between humans and parasitic roundworms.

• Describe the anatomy of a rotifer.

Section 2 Nematoda and RotiferaChapter 34

Phylum Nematoda

• The phylum Nematoda is made up of roundworms with long, slender bodies that taper at both ends.

• Roundworms are among several phyla of pseudocoelomates.

• Roundworms have a digestive tract with two openings.

• Most roundworms have separate sexes and are covered by a protective cuticle.

• Most species are free-living; some are parasites.

Section 2 Nematoda and RotiferaChapter 34

Phylum Nematoda, continued

Ascaris• The genus Ascaris infects pigs, horses, and humans. • The eggs enter hosts through contaminated food or water,

develop into larvae in the intestines, and can infect the lungs.• The eggs are spread in the hosts’ feces.

Hookworms• Hookworms are intestinal parasites that feed on blood.• The eggs produce larvae in soil, and the larvae enter hosts

through the feet.• Hookworms infect about one billion people worldwide.

Section 2 Nematoda and RotiferaChapter 34

Phylum Nematoda, continued

Trichinella• The genus Trichinella infects humans and other mammals.• Adults live in intestines and larvae form cysts in muscles. • People usually become infected from undercooked pork.• Infection causes the disease trichinosis.

Other Parasitic Roundworms• Pinworms, genus Enterobius, are common parasites of humans.

They do not cause any serious disease. • Filarial worms infect many people in tropical countries. The most

dangerous ones infect the lymphatic system and may cause elephantiasis.

Chapter 34

Exploration of a Roundworm

Section 2 Nematoda and Rotifera

Section 2 Nematoda and RotiferaChapter 34

Phylum Rotifera

• Members of the phylum Rotifera are called rotifers. • Most rotifers are tiny, transparent, free-living animals

that live in fresh water.• Some can survive without water for long periods.• Although tiny, they are truly multicellular and have

specialized organ systems.• They use the crown of cilia around their mouth to

sweep food into the mastax.• The digestive, reproductive, and excretory systems

empty into the cloaca.• Some species reproduce by parthenogenesis.