Chapter 34Flatworms (Platyhelminthes),

Roundworms (Nematoda) & Rotifera

Photos from: http://www.101science.com/Invertebrates.htm

Please note that in addition to these notes, we will refer to overhead transparencies & diagrams on pages 690, 692, 693, 694, 696 & 698. You are responsible for that content as well.

I. Flatworms (Platyhelminthes)A. Structure • Bilateral symmetry

– Are the simplest of animals to have bilateral symmetry – Body flattened, leaf or ribbon-like (dorsal-ventral)

• 3 tissues layers– Ectoderm, – Mesoderm– Endoderm

• Acoelomate (No body cavity) – solid body- – completely filled in with tissue.

B. Systems

1. Digestive System-only 1 opening for gastrovascular cavity

-sac body plan, with mouth serving as both a mouth and anus (there is no anus).

(PARTS DIFFER- see each class)-in planarian, pharynx with highly branched cavity

-flukes, sucker mouth with either digestive cavity or just absorb nutrients from host (reduced digestive system)

-in tapeworms- no mouth or gastrovascular cavity -

just absorb nutrients directly from host digestive tract

Excretory System

Platyhelminthes are the simplest animals to have a dedicated excretory system.

In planarians & flukes:

-Flame cells -a specialized excretory cell to eliminate excess water -function like a kidney, removing waste materials.

-are "cup-shaped", with cilia covering the inner surface The beating cilia resemble a flame, giving the cell its name.

-Excretory tubules -Beating of the cilia moves liquid through the excretory tubule & pores-out of the organism.

2. & 3. No true Circulatory or Respiratory systems

-cells exchange oxygen & CO2 directly from environment to cells

Cephalization: Planarians have a definite head with sense organs (Eyespots & other sensory cells)

3. Nervous System- cerebral ganglia serves as a simple brain- transmits signals down “ladder-like” nerve network. Can respond to light, chemicals, touch & water currents in the environment

4. Reproductive System- planarians are hermaphrodites- both male & female organs. Simultaneously fertilize each other.

4 Classes of Flatworms:

-Class Turbellaria (free-living flatworm predators)

-Class Trematoda & Monogenea (flukes)

-Class Cestoda (tapeworms). flukes and tapeworms are adapted to a parasitic mode of life

COOL THING ABOUT FLATWORMS:– If cut- they will re-grow parts missing (regeneration)

Class Turbellaria• 4, 500 species in ocean or fresh water• Scavenger lifestyle, eat with pharynx• Flame cells (cilia- aids excretion)• Neural- cerebral ganglia & eyespot sensory

equipment. “Spade” shaped head.• Example is Dugesia, freshwater (family Planariidae)

• the term "planaria" is most often used as a common name, but It is

also the name of a genus within the family Planariidae

www8.nos.noaa.gov/.../index.aspx?letter=e http://www.discoveryscope.net/images/circleworm.gif

• Embryonic

development

of a flatworm

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. • External surface is covered by a protective layer

called the tegument.

Reproduction &Life Cycle of Flukes

• Most flukes have highly developed reproductive systems and are hermaphroditic.

• Fertilized eggs stored in uterus, then released. • Complicated life cycles

– involve more than one host species.

• For example, the trematode blood flukes of the genus Schistosoma – use humans as a primary hosts – snails as intermediate hosts. – They can cause schistosomiasis.

Life Cycle of Schistosoma

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

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.

Life cycle of a human beef tapeworm, Taenia saginatus

• A man eats a poorly cooked piece of beef containing the encysted bladderworm (tapeworm cyst) in the skeletal muscle tissue.

• Tapeworm attaches to the intestinal wall by its anterior end called the scolex. (anchors it to the host's intestinal wall by four suckers.)

• Note: Some early diet pills actually contained a tapeworm cyst enclosed in a gelatin capsule, before they were banned by the Food & Drug Administration. You could take the pill and eat as much as you want without gaining weight! You were actually feeding a large tapeworm which could be very dangerous.

• Grows by budding as the scolex produces flattened segments called proglottids. – A human tapeworm may contain hundreds of proglottids and be

several feet in length. • Tapeworms compete with their host for nutrients. • Large tapeworms may obstruct food passages.

• Human tapeworms may be several feet long.

• San Diego Clinic case- A man noticed a long white ribbon in his feces and thought he had swallowed his pajama draw strings during the night. The white ribbon turned out to be a tapeworm!

Story & photo from: http://waynesword.palomar.edu/trnov01.htm

Beef Tapeworm life cycle

II. Nematoda (Roundworms)• Nonsegmented round worms that include many common

parasites• Long, slender bodies that taper at both ends.• Most species are free-living; some are parasites.

• A. Structure-Bilateral symmetry

-3 tissue layers-Pseudocoelomates • body cavity not completely lined by mesoderm• where organs are found • can serve as a hydrostatic skeleton.

• Roundworms have a digestive tract with two openings.

• Most roundworms have separate sexes

• are covered by a protective cuticle.

Most roundworms are free living • See examples of parasites:

Ascaris (See page 696 textbook)• 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 (also p. 696)• 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.

Trichinella (SEE page 697 textbook)• 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.

III. Rotifera (see page 698)

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.

Digestive:• They use the crown of cilia around their mouth to

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

empty into a common chamber called the cloaca. (see diagram p 698)

Reproduction:• Some species reproduce by parthenogenesis.

A SPECIAL KIND OF ASEXUAL REPRODUCTION- UNFERTILIZED EGGS DEVELOP INTO ADULT FEMALES)

Other species reproduce sexually.

Questions1. What does a planarian use its pharynx for?

A. feedingB. movementC. reproductionD. to respond to light

2. Where do blood flukes of the genus Schistosoma reproduce asexually?F. in waterG. inside a snailH. inside a cow’s intestineJ. inside a human’s blood vessels

3. What does a tapeworm use its scolex for?A. to reproduceB. to attach itself to its hostC. to eliminate excess waterD. to force food into its mouth

4. Which of the following is true of most rotifers?F. They are parasitic.G. They live in the soil.H. They feed with the help of cilia.J. They have a gastrovascular cavity.