Lesson Activity on Sponges,
Cnidarians, Flatworms and
Roundworms
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Sponges
Cnidarians
Flatworms
Roundworms
1 Sponges – phylum Porifera
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Asymmetrical
Invertebrates
No tissues, organs or organ systems
Over 5000 species in a variety of sizes, shapes and
colors
Sponges Are Pore-Bearers
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Porifera = pore bearer
Almost all live in shallow areas of the ocean
Mainly sessile
Sessile - it is permanently attached to a surface
for all of its adult life
Get food by filter feeding (filter small particles of
food from the water as it passes by or through
some part of the organism)
Cell Organization in Sponges
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Sponges have different types of cells that perform
different functions
Sponge embryos do not develop endoderm or
mesoderm, so their cells are not organized into
tissues
Endodrem and Mesoderm: The germ layers in the
process by which the embryo is formed and
develops
Why Are Sponges Important?
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Can be separated into individual types of cells but
if left alone (over time) the cells will reorganize
back into a sponge
Believed to have evolved from colonial,
flagellated protists
Demonstrate the major evolutionary step between
unicellular life to a division of labor among groups
of organized cells
Reproduction in Sponges
Reproduce both sexually and asexually
 Asexually through fragmentation or
external buds
 Buds may break off and float away to
become new animals or they may remain
attached and form a colony
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Sexual Reproduction in
Sponges
All are hermaphrodites
 Hermaphrodites can produce both eggs and
sperm AT different times
 It increases the likelihood of fertilization in
sessile animals
 Cannot self-fertilize
 Can use either internal or external
fertilization
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Internal Fertilization
Internal fertilization – sperm is brought
into the body and fertilization takes place
inside the organism
 Eggs remain inside the sponge
 Collar cells collect sperm and transfer it to
the amoebocytes
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External Fertilization
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External fertilization – sperm and eggs are
released into the water and fertilization occurs out
in the water
Some sponges in temperate zones produce
gemmules which are like seeds and lie dormant
over the winter and grow into new sponges in the
spring
2.Cnidarians (the C is silent)
What are Cnidarians?
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Marine invertebrates
9,000 species
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Include jellyfish, corals, sea anemones and hydras
Found worldwide, but mainly in warmer oceans
Characteristics of Cnidarians
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Radial Symmetry
2 cell layers (the ectoderm and the endoderm) with one
body opening
Ectoderm becomes a protective outer layer of cells
The endoderm is internal and is adapted mainly to aid in
digestion
Cnidarians Body Forms
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2 basic body forms that occur at
different stages of their life cycle
Polyp - Tube shaped body and a mouth
surrounded by tentacles
 Dominant in hydras (spends most of its
life in this form)
 Corals and sea anemones only have
this stage
Medusa- body shaped like an
umbrella with tentacles hanging down
 Dominant in jellyfish
Body Systems
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Have simple nervous systems and other tissues
Nerve net – conducts nerve impulses from all
parts of the body
There is no brain
Both cell layers have cells that can contract like
muscles
Simple digestive system
Digestion in Cnidarians
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Predators that capture or poison their prey with
nematocysts
Nematocyst - capsule that contains a coiled,
thread like tube that may contain a toxin
Digestion involves enzymes and cells adapted
for this purpose
Digestion takes place in the gastrovascular cavity
Undigested materials are ejected back out the
mouth
Reproduction in Cnidarians
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Sexual reproduction usually occurs in the medusa
stage (unless there is none)
Asexual may occur in either the polyp or medusa
stage
Technically not alternation of generations like in
plants because both stages are diploid
Common Reproductive Cycle in
Cnidarians
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Male medusae release sperm
Female medusae release eggs
Fertilization occurs
The zygotes develops into an embryo and then
into a larva
The free swimming larva settles down and
develops into a polyp
The polyp reproduces asexually to form male and
female medusae
Respiration
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Oxygen enters cells directly
Because of its body plan, no cell is ever far from
water
Oxygen dissolved in the water diffused directly
into the cells
Carbon dioxide and other wastes diffuse directly
out and into the water
Diversity of Cnidarians
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Most of the 9000 species belong to one of 3
classes
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Hydrozoans
Scyphozoans
Anthozoans
Most Hydrozoans
Form Colonies
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Class Hydrozoa has 2 groups – 1. hydroids (hydra)
and 2. Siphonophores (Portuguese Man-O-War)
Most hydroids are branching polyp colonies
formed by budding
Siphonophores are floating or
swimming colonies of medusae
Each individual in Siphonophores
colonies has a different function, but
they all function together for the
survival of all
Scyphozoans are the Jellyfish
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Jellyfish
Medusa stage is dominant
Can be found everywhere in
the
oceans and as deep as1000 meters
The gastrovascular cavity has 4 internal divisions
Range in size from microscopic to
more than a meter
Anthozoans Build
Coral Reefs
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Found in tropical, temperate and arctic seas
Cnidarians that exhibit only the polyp form
Have many divisions in their gastrovascular cavity
Corals live in colonies
Sea Anemones live as individual animals
Coral Reefs
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Corals secrete a calcium carbonate ‘skeleton’ that
remains after it dies forming reefs
Reefs grow very slowly
Coral reefs are very sensitive
towards the changes in
temperature and water level
Corals
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Most of Coral form mutual relationships with
photosynthetic protists which offer the corals
oxygen and food and use the carbon dioxide and
wastes from the corals
These protists are primarily responsible for the
bright colors of coral reefs
If these protists leave the corals, the corals die
Origins of Cnidarians and
Sponges
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Sponges represent the oldest animal phylum
dating from 700 million years ago
Thought to have evolved directly from flagellated
protists similar to the collar cells of today
Cnidarians first appear about 630 million years
ago.
We have little evidence for cnidarians as they are
soft bodied and do not fossilize well
Believed that cnidarians evolved from protists
3. Flatworms – phylum
Plathelminthes
What Is a Flatworm?
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Acoelomates - thin solid
Bilateral symmetry
Approx. 14,500 species
Live in salt and freshwater
Planarians (Class Turbellaria)
Tapeworms (Class Cestoda)
Flukes(Class Trematoda)
bodies
Feeding and Digestion in
Planarians
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Feed on dead or slow moving organisms
Pharynx – tube-like muscular organ that a
planarian extends from its mouth to feed
Enzymes begin digesting the food outside the
animal’s body
Food is digested by individual cells
Cells lining the digestive tract obtain food by
phagocytosis
Nervous Control in Planarians
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Some have a nerve net, other have the beginnings
of a central nervous system (CNS)
CNS in planarians
2 eyespots
2 nerve cords that run the length of the body
Brain-like structure called a ganglion which sends
messages to/from the eyespots and along the nerve
cords
Reproduction in Planarians
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Most are hermaphrodites
During sexual reproduction individual flatworms
exchange sperm
Fertilization is internal
Zygotes (fertilized eggs) are released into the
water to hatch
Planarians Can Also Reproduce
Asexually
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If it is damaged it can regenerate (grow back) new
body parts
Missing body parts are replaced through mitosis
If cut in half the head end will grow a new tail and
the tail end will grow a new head
Feeding and Digestion in
Parasitic Flatworms
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Parasites are adapted to living inside a host and
obtaining nutrients from their host
Have mouthparts with hooks to hold the worm
inside its host
Did not evolve complex nervous or muscular
systems because they didn’t need them
Tapeworms (Class Cestoda)
Parasitic
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Made up of a head (scolex) and repeating sections
(proglottids)
The proglottids are detachable and each contains
nerves, muscles, flame cells, as well as male and
female reproductive organs
Each proglottid can have up to100,000 eggs
A tapeworm can have as many
as 2,000 proglottids
Flukes(Class Trematoda)
Flukes are parasitic flatworms that embed
themselves in the internal organs of
vertebrates
 Feeds on cells, blood and other fluid of the
host
 Blood flukes can
cause schistosomiasis
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4. Roundworms
phylum Nematoda
What is a Roundworm?
Varied habitat - live in soil, saltwater and
freshwater environments
 Most are parasitic
 Smaller than tapeworms with a thick outer
covering that keeps them from being
digested
 Tapered at both ends
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Muscular and Digestive
Systems of Roundworms
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Have a pseudocoelom 2 body openings (mouth
and anus)
Simplest animals with a tubelike digestive system
Have pairs of lengthwise muscles
Lack circular muscles
Move in a thrashing motion
Diversity of Roundworms
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Free-living species have well developed
eyespots/sensory organs
Some species are parasitic to plants and fungi
Can form symbiotic relationships with bacteria
50 species are human parasites (including
hookworm and pinworm)
Roundworms infect humans by
several methods
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Can go through the host’s:
 Digestive tract (Ascarsis)
 Through the skin (Hookworms)
 Through undercooked food (pigs infected with
Trichinella)
Roundworm parasites of other
organisms
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Nematodes infect pine trees, cereal crops and food
plants
Attracted to plant roots
1200 species cause disease in plants
Soil nematodes invade
roots for food