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Photos from: http://www.101science.com/Invertebrates.htm
Chapter 34
Flatworms (Platyhelminthes),
Roundworms (Nematoda)
& Rotifera
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 caseA 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, freeliving 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 REPRODUCTIONUNFERTILIZED EGGS DEVELOP INTO ADULT FEMALES)
Other species reproduce sexually.
Questions
1. What does a planarian use its pharynx for?
A. feeding
B. movement
C. reproduction
D. to respond to light
2. Where do blood flukes of the genus Schistosoma
reproduce asexually?
F. in water
G. inside a snail
H. inside a cow’s intestine
J. inside a human’s blood vessels
3. What does a tapeworm use its scolex for?
A. to reproduce
B. to attach itself to its host
C. to eliminate excess water
D. 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.