Chapter 26: Sponges, Cnidarians, and
Unsegmented Worms
Section 1: Introduction to the
Animal Kingdom
Introduction to the Animal Kingdom
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The animal kingdom is the most diverse in
form
Each animal performs the essential
functions of life in its own special way
Two divisions that we will use to separate
the animal kingdom are vertebrates and
invertebrates
 Vertebrates have a backbone
 Invertebrates have no backbone
What Is an Animal?
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All animals share certain basic
characteristics
Animals are heterotrophs (they do NOT
make their own food)
Instead, they obtain the nutrients and
energy they need by feeding on organic
compounds that have been made by other
organisms
What Is an Animal?
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Animals are multicellular, which means
that their bodies are composed of more
than one cell
Animal cells are also eukaryotic – they
contain a nucleus and membrane-enclosed
organelles
An animal is a multicellular
eukaryotic heterotroph whose cells
lack cell walls
Cell Specialization and Division of
Labor
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The bodies of animals contain many types
of specialized cells
Each specialized cell has a shape, physical
structure, and chemical composition that
make it uniquely suited to perform a
particular function within a multicellular
organism
For this reason, groups of specialized cells
carry out different tasks for the organism –
division of labor
What Animals Must Do to Survive
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In order to survive, animals must be able
to perform a number of essential functions
For each animal group we study in the
next several chapters, we will examine
these functions and describe the cells,
tissues, organs, and organ systems that
perform them
Feeding
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Animals have evolved a variety of ways to feed
Herbivores eat plants
Carnivores eat animals
Parasites live and feed either inside or attached
to outer surfaces of other organisms, causing
harm to the host
Filter feeders strain tiny floating plants and
animals from the water around them
Detritus feeders feed on tiny bits of decaying
plants and animals
Respiration
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Living cells consume oxygen and give off
carbon dioxide in the process of cellular
respiration
Entire animals must respire, or breathe, in
order to take in and give off these gases
Small animals that live in water or in moist
soil may respire through their skin
Respiration
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For large active animals, however,
respiration through the skin is not efficient
The respiratory systems these animals
have evolved take many different forms in
adaptations suited to different habitats
Internal Transport
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Some aquatic animals can function without
an internal transport system
But once an animal reaches a certain size,
it must somehow carry oxygen, nutrients,
and waste products to and from cells deep
within its body
Many multicellular animals have evolved a
circulatory system in which a pumping
organ called a heart forces a fluid called
blood through a series of blood vessels
Excretion
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Cellular metabolism produces chemical
wastes such as ammonia that are harmful
and must be eliminated
Small aquatic animals depend on diffusion
to carry wastes from their tissues into the
surrounding water
But larger animals, both in water and on
land, must work to remove poisonous
metabolic wastes
Response
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Animals must keep watch on their
surroundings to find food, spot predators,
and identify others of their own kind
To do this, animals use specialized cells
called nerve cells, which hook up together
to form a nervous system
Response
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Sense organs, such as eyes and ears,
gather information from the environment
by responding to light, sound,
temperature, and other stimuli
The brain, which is the nervous system’s
control center, processes the information
and regulates how the animal responds
The complexity of the nervous system
varies greatly in animals
Movement
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Some animals are sessile, which means
that they live their entire adult lives
attached to one spot
But many animals are motile, which means
that they move around
To move, most animals use tissues called
muscles that generate force by contracting
In the most successful groups of animals,
muscles work together with a skeleton, or
the system of solid support in the body
Movement

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Insects and their relatives wear their
skeletons on the outside of their bodies
 exoskeletons
Reptiles, birds, and mammals have their
skeletons inside their bodies


endoskeletons
We call the combination of an animal’s
muscles and skeleton its musculo –
skeletal system
Reproduction
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Animals must reproduce or their species
will not survive
Some animals switch back and forth
between asexual and sexual reproduction
Many animals that reproduce sexually bear
their young alive
Reproduction

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Others lay eggs
The eggs of some species hatch into baby
animals that look just like miniature adults
These baby animals increase in size but do
not change their overall form
 Direct development
Reproduction
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In other species, eggs hatch into larvae,
which are immature stages that look and
act nothing like the adults
As larvae grow, they undergo a process
called metamorphosis in which they
change shape dramatically
 Indirect development
Trends in Animal Evolution
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The levels of organization become higher
as animals become more complex in form
The essential functions of less complex animals
are carried out on the cell or tissue level of
organization
As you move on to more complex animals, you
will observe a steady increase in the number of
specialized tissues
You will also see those tissues joining together
to form more and more specialized organs and
organ systems
Trends in Animal Evolution
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Some of the simplest animals have radial
symmetry; most complex animals have
bilateral symmetry
Some of the simplest animals have body parts
that repeat around an imaginary line drawn
through the center of their body
 Radial symmetry
 Animals with radial symmetry never have
any kind of real “head”
 Many of them are sessile, although some
drift or move in a random pattern
Trends in Animal Evolution
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Most complex invertebrates and all
vertebrates have body parts that repeat on
either side of an imaginary line drawn
down the middle of their body
One side of the body is a mirror image of
the other
These animals are said to have bilateral
symmetry
Trends in Animal Evolution

Animals with bilateral symmetry have
specialized front and back ends as well
as upper and lower sides
 Anterior = front end
 Posterior = back end
 Dorsal = upper side
 Ventral = lower side
Trends in Animal Evolution
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More complex animals tend to have a
concentration of sense organs and
nerve cells in their anterior (head)
end
This gathering of sense organs and nerve
cells into the head region is called
cephalization
Nerve cells in the head gather into clusters
that process the information gathered by
the nervous system and control responses
to stimuli
Trends in Animal Evolution

Small clusters of nerve cells are called
ganglia
 In the most complex animals, large
numbers of nerve cells gather together
to form larger structures called brains
Chapter 26: Sponges, Cnidarians, and
Unsegmented Worms
Section 2: Sponges
Sponges
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Sponges are among the most ancient of all
animals that are alive today
Most sponges live in the sea, although a
few live in freshwater lakes and streams
Sponges inhabit almost all areas of the sea
– from the polar regions to the tropics and
from the low-tide line down into water
several hundred meters deep
Sponges
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Sponges belong to the phylum Porifera
 Literally means pore-bearers
 Tiny openings all over their body
Sponges were once thought to be plants
Sponges are sessile and show little
detectable movement
Sponges are heterotrophic, have no cell
walls, and contain several specialized cell
types that live together
Sponges

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Sponges are very different from other animals
Sponges have nothing that even vaguely
resembles a mouth or gut, and they have
no specialized tissues or organ systems
Most biologists believe that sponges
evolved from single-celled ancestors
separately from other multicellular
animals
The evolutionary line that gave rise to sponges
was a dead end that produced no other groups
of animals
Form and Function in Sponges
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Very simple body plan
The body of a sponge forms a wall around
a central cavity
In this wall are thousands of pores
A steady current of water moves through
these pores into the central cavity
This current is powered by the flagella of
cells called collar cells
Form and Function in Sponges
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The water that gathers in the central
cavity exits through a large hole called the
osculum
The current of water that flows through
the body of a sponge delivers food and
oxygen to the cells and carries away
cellular waste products
The water also transports gametes or
larvae out of the sponge’s body
Form and Function in Sponges
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Many sponges manufacture thin, spiny
spicules that form the skeleton of the
sponge
A special kind of cell called an amebocyte
builds the spicules from either calcium
carbonate or silica
These spicules interlock to form beautiful and
delicate skeletons
The softer but stronger sponge skeletons that
we know as natural bath sponges consist of
fibers of a protein called spongin
Form and Function in Sponges
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Sponges are filter feeders that sift
microscopic particles of food from the water
that passes through them
All digestion in sponges is intracellular; it
takes place inside cells
The water flowing through a sponge
simultaneously serves as its respiratory,
excretory, and internal transport system
As water passes through the body wall,
sponge cells remove oxygen from it and
give off carbon dioxide to it
Form and Function in Sponges
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The water that flows through the body of
a sponge also plays a role in sexual
reproduction
Although eggs are kept inside the body
wall of a sponge, sperm are released into
the water flowing through the sponge and
are thus carried out into the open water
If those sperm are taken in by another
sponge, they are picked up by amebocytes
and carried to that sponge’s eggs, where
fertilization occurs
Form and Function in Sponges
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The zygote that results develops into a larva that
swims and can be carried by currents for a long
distance before it settles down and grows into a new
sponge
Sponges can also reproduce asexually
Faced with cold winters, some freshwater sponges
produce structures called gemmules
 Sphere-shaped collections of amebocytes
surrounded by a tough layer of spicules
 Can survive long periods of freezing temperatures
and drought
 When conditions become favorable, gemmules
grow into new sponges
Form and Function in Sponges

Sponges can also reproduce asexually by
budding
 In this process, part of a sponge simply
falls off the parent and grows into a new
sponge
 Remarkable powers of regeneration
How Sponges Fit into the
World
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Sponges provide housing for many other
marine animals
Sponges are also involved in symbiotic
relationships with other organisms
Humans have used the dried and cleaned
bodies of some sponges in bathing
Some chemicals that sponges secrete are
being used as powerful antibiotics that are
used to treat bacteria and fungi
Chapter 26: Sponges,
Cnidarians, and Unsegmented
Worms
Section 3: Cnidarians
Cnidarians

The phylum Cnidaria includes many
animals with brilliant colors and unusual
shapes
 Jellyfish, sea anemones, etc.
 These beautiful and fascinating
animals are found all over the world,
but most species live only in the sea
What is a Cnidarian?
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Cnidarians are soft-bodied animals with
stinging tentacles arranged in circles
around their mouth
Some cnidarians live as single individuals
Others live as groups of dozens or even
thousands of individuals connected into a colony
All cnidarians exhibit radial symmetry and have
specialized cells and tissues
Many cnidarians have life cycles that include two
different-looking stages, the sessile flowerlike
polyp and the motile bell-shaped medusa
Some cnidarians, such as sea nettles
and sea anemones, are solitary.
Others, such as gorgonian coral
polyps, are colonial.
What is a Cnidarian?


Both polyps and medusa have a body wall that surrounds
an internal space called the gastrovascular cavity
 This is where digestion takes place
The body wall consists of three layers:
 Epidermis
 Layer of cells that covers the outer surface of the
cnidarian’s body
 Mesoglea
 Located between the epidermis and the gastroderm
 Gastroderm
 Layer of cells that covers the inner surface, lining
the gastrovascular cavity
Form and Function in
Cnidarians

Almost all cnidarians capture and eat small
animals by using stinging structures called
nematocysts, which are located on their
tentacles
 Poison-filled sac containing a tightly coiled
spring loaded dart
 When an animal touches a nematocyst, the
dart uncoils and buries itself into the skin of
the animal
 Paralyzes or kills the prey
Form and Function in
Cnidarians
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From here, the cnidarian’s tentacles push the
food through the mouth and into the
gastrovascular cavity
There the food is gradually broken up into tiny
pieces
These food fragments are taken up by special
cells in the gastroderm that digests them further
The nutrients are then transported throughout
the body by diffusion
Any materials that cannot be digested are passed
back out through the mouth, which is the only
opening in the gastrovascular cavity
Form and Function in
Cnidarians
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Because most cnidarians are only a few cell
layers thick, they have not had to evolve
many complicated body systems in order to
survive
There is no organized internal transport
network or excretory system in cnidarians
Cnidarians also lack a central nervous system
and anything that could be called a brain
They have simple nervous systems called
nerve nets
 Concentrated around the mouth
Form and Function in
Cnidarians
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Cnidarians lack muscle cells that most
other animals use to move about
Many of the epidermal cells in cnidarians
can change shape when stimulated by the
nervous system
Cnidarian polyps can expand, shrink, and
move their tentacles by relaxing or
contracting these epidermal cells
Form and Function in
Cnidarians
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Most cnidarians can reproduce both sexually and
asexually
Polyps can produce new polyps asexually by budding
When medusae mature, they reproduce sexually by
releasing gametes into the water
Fertilization occurs either in open water or inside an
egg-carrying medusa
The zygote grows into a ciliated larva that swims
around for some time
Later, the larva settles down, attaches to a hard
surface, and changes into a polyp that begins the
cycle again
Hydras and Their Relatives
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Class Hydrozoa is made up of cnidarians that
spend most of their lives as polyps, although
they usually have a short medusa stage
Most hydrozoan polyps grow in branching sessile
colonies
 Range in length from a few centimeters to
more than a meter
 Specialized polyps perform particular functions
 Feeding
 Reproduction
 Defense
Hydras and Their Relatives
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Most common are the hydras
Hydras can reproduce either asexually by
budding or sexually by producing eggs and
sperm in their body walls
In most species of hydras, the sexes are
separate
However, a few species are hermaphrodites
 An individual that has both male and
female reproductive organs and produces
both sperm and eggs
Hydras and Their Relatives

One unusual hydrozoan in the Portuguese manof-war
 Form floating colonies that contain several
polyps
 One polyp forms a balloon-like float that keeps
the colony on the surface
 Some of the polyps produce long stinging
tentacles that paralyze and capture prey
 Some polyps digest the food held by tentacles
Jellyfish
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Class Scyphozoa
Go through the same life-cycle stages as
hydrozoans
Some jellyfish, such as the lion’s mane, often grow
up to 2 meters in diameter
The largest jellyfish ever found was more than 3.6
meters in diameter and had tentacles more than 30
meters long
The nematocysts of most jellyfish are harmless to
humans, but a few can cause painful stings
 One tiny Australian jellyfish has a toxin powerful
enough to cause death in 3 – to 20 minutes
Sea Anemones and Corals
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Class Anthozoa
Most beautiful and ecologically important
invertebrates
Have only the polyp stage in their life cycle
Adult polyps reproduce sexually by producing
eggs and sperm that are released into the water
The zygote grows into a ciliated larva that settles
to the ocean bottom and becomes a new polyp
Many anthozoans also reproduce asexually by
budding
Sea Anemones and Corals
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Sea anemones are solitary polyps that live
in the sea from the low-tide line to great
depths
Although they can catch food with the
nematocysts on their tentacles, many
shallow-water species depend heavily on
their photosynthetic symbionts
Some sea anemones can grow up to a
meter in diameter
Sea Anemones and Corals
Corals grow in shallow tropical water
around the world
 Corals produce skeletons of calcium
carbonate or limestone
 Most corals are colonial
 As a coral colony grows, new polyps are
produced by budding

Sea Anemones and Corals
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Coral colonies grow very slowly, but they may
live for hundreds, or even thousands, of years
Together, countless coral colonies produce
huge structures called coral reefs
Some of these reefs are enormous and
contain more rock and living tissue than even
the largest human cities
The Great Barrier Reef off the coast of
Australia is more than 2000 km long and
some 80 km wide
How Cnidarians Fit into the
World
Certain fish, shrimp, and other small
animals live among the tentacles of
large sea anemones
 Corals and the reefs provide shelter for
thousands of species of marine life
 Reefs protect the land from erosion
 Jewelry and decorations
 Medical research

Chapter 26: Sponges,
Cnidarians, and Unsegmented
Worms
Section 4: Unsegmented Worms
Unsegmented Worms

Unsegmented worms have bodies that
are not divided into special segments
 Phylum Platyhelminthes
 Consists of simple animals called
flatworms
 Phylum Nematoda
 Consists of long, thin worms called
roundworms
Flatworms
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The members of the phylum
Platyhelminthes are the simplest animals
with bilateral symmetry
Most members of this phylum exhibit
enough cephalization, or development of
the anterior end, to have what we call a
head
Many flatworms are no more than a few
millimeters thick, although they may be up to 20
meters long
Flatworms have more developed organ systems
than either sponges or cnidarians
Form and Function in
Flatworms
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Flatworms feed in either of two very different
ways
Worms may be carnivores that feed on tiny
aquatic animals
Free-living flatworms have a gastrovascular
cavity with one opening at the end of a muscular
tube called a pharynx
They use the pharynx to suck food into the
gastrovascular cavity
The gastrovascular cavity forms an intestine with
many branches along the entire length of the
worm
Form and Function in
Flatworms
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In the intestines, enzymes help break
down the food into small particles
These particles are taken inside the cells
of the intestinal wall, where digestion is
completed
Like Cnidarians, flatworms expel
undigested material through the mouth
Form and Function in
Flatworms
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Many other flatworms are parasites that feed
on blood, tissue fluids, or pieces of cells
inside the body of their host
In many parasitic flatworms, the digestive
tract is simpler than in free-living forms
Tapeworms, which live within the intestines of
their host, do not have any digestive tract at
all
They have hooks and/or suckers with which
they latch onto the intestinal wall of the host
Form and Function in
Flatworms
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From this position, they can simply absorb
the food that passes by – food that has
already been broken down by the host’s
digestive enzymes
Flatworms lack any kind of specialized
circulatory or respiratory system
Freshwater flatworms such as planarians
have structures called flame cells that help
them get rid of extra water
Form and Function in
Flatworms
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Free-living flatworms have nervous systems
that are much more developed than those of
cnidarians and sponges
They have a definite head in which a simple
brain is located
One or more long nerve cords run from the
brain down the length of the body on either
side
Many flatworms have one or more pairs of
light-sensitive organs called ocelli, or
eyespots
Form and Function in
Flatworms
The nervous system of free-living
flatworms allows them to gather
information from their environment –
information that they use to locate food
and to find dark hiding places
 Parasitic flatworms often do not have
much of a nervous system

Form and Function in
Flatworms

Free-living flatworms usually use two
means of locomotion at once
 Cilia on their epidermal cells help
them glide through the water
 Muscle cells controlled by the nervous
system allow them to twist and turn
so that they are able to react to
environmental conditions
Form and Function in
Flatworms
 Reproduction
in free-living
flatworms can be either sexual or
asexual
 Most free-living flatworms are
hermaphrodites
 The eggs hatch within a few weeks
Planarians
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The free-living flatworms belong to the
class Turbellaria
Most familiar members of this class are
planarians
Turbellarians vary greatly in color, form,
and size
Although most Turbellarians are less that 1
cm in length, some giant land planarians,
which are found in moist tropical areas,
can attain lengths of more than 60 cm
Flukes
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Class Trematoda contains parasitic flatworms
known as flukes
Most flukes are internal parasites that infect
the blood and organs
These flukes have complicated life cycles that
involve at least two different host animals
Blood flukes are found primarily in Southeast
Asia, North Africa, and other tropical areas
Humans are the primary hosts of blood flukes
Flukes
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Most flukes are hermaphrodites and undergo
sexual reproduction in a manner similar to
that of free-living flatworms
Flukes produce many more eggs than freeliving flatworms
Blood flukes lay so many eggs that the tiny
blood vessels of the host’s intestine break
open
The broken blood vessels leak both blood and
eggs into the intestine
Flukes
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The eggs are not digested by the host and
thus become part of the feces
In developed countries, where there are
toilets and proper sewage systems, these
eggs are usually destroyed in the sewage
treatment process
But in many undeveloped parts of the world,
human wastes are simply tossed into streams
or even used as fertilizer
Flukes
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Once the fluke eggs get into the water, they
hatch into swimming larvae
When these larvae find a snail of the correct
species, they burrow inside it and digest its
tissues
The snail is an intermediate host for the fluke
In the intermediate host, the flukes
reproduce asexually
Flukes
The resulting new worms break out of
the snail and swim around in the water
 If they find a human, the worms bore
through the skin and eat their way to
the blood vessels
 In the blood, the get carried around
through the heart and lungs to the
intestine, where they live as adults

Flukes
 People
infected with blood flukes get
terribly sick
 They become weak and often die –
either as a direct result of the fluke
infection or because they cannot
recover from other diseases in their
weakened condition
Tapeworms

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Members of the class Cestoda are long, flat
parasitic worms that live a very simple life
They have a head called a scolex on which
there are several suckers and a ring of
hooks
These structures attach to the intestinal
walls of humans and other animals
Adult human tapeworms can be up to 18
meters long
Tapeworms almost never kill their host
Tapeworms



Behind the scolex of the tapeworm is a
narrow neck region that is constantly dividing
to form many proglottids, or sections, that
make up most of the body of the tapeworm
The youngest and smallest proglottids are at
the anterior end of the tapeworm
Male and female reproductive organs are
contained in the proglottids
Tapeworms


If food or water contaminated with
tapeworm eggs is consumed by cows, pigs,
fish, or other intermediate hosts, the eggs
enter the intermediate host and hatch into
larvae
These larvae grow for a time and then
burrow into the muscle tissue of the
intermediate host and form a dormant
protective stage called a cyst
Tapeworms
 If
a human eats raw or incompletely
cooked meat containing these cysts,
the larvae become active within the
human host
 Once inside the intestine of a new
host, they latch onto the intestinal
wall and grow into adult worms
Roundworms

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

Members of the phylum Nematoda, which
are known as roundworms, are among
the simplest animals to have a digestive
system with two openings – a mouth and
an anus
Food enters through the mouth, and
undigested food leaves through the anus
Roundworms may be the most numerous of all
multicellular animals
A single rotting apple can contain as many as
90,000 roundworms
Form and Function in
Roundworms

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Most roundworms are free-living
All roundworms have a long tube-shaped
digestive tract with openings at both ends
Any material in the food that cannot be
digested leaves through an opening called
the anus
Roundworms breathe and excrete their
metabolic wastes through their body walls
They have no internal transport system
Form and Function in
Roundworms
 Roundworms
have simple nervous
systems
 They have several ganglia in the
head region but no definite brain
 Roundworms reproduce sexually
 Fertilization takes place inside the
body of the female
How Unsegmented Worms Fit into the
World
 Do
not have a lot of positive
influence on humans
 Responsible for some of the most
painful and horrific diseases known
 Hookworm
 Eye worm
 Trichinosis