sponges and cnidarians

digestive and nervous systems

Cnidarians have an incomplete digestive system with a single opening. The opening is surrounded by tentacles, which are covered with nematocyst cells and used to capture prey. Digestion takes place in the digestive cavity. Nutrients are absorbed and gases are exchanged through the cells lining this cavity. Fluid in the cavity supports and stiffens the cnidarian body. Cnidarians have a simple nervous system. It consists of a net of nerves that can sense touch. You can see a sketch of the nerve net in a hydra in Figure 12.9. Some cnidarians also have other sensory structures. For example, jellyfish have light-sensing structures and gravity-sensing structures.

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how cnidarians reproduce

Cnidarians in the polyp form usually reproduce asexually. One type of asexual reproduction in polyps leads to the formation of new medusae. Medusae usually reproduce sexually with sperm and eggs. Fertilization forms a zygote. The zygote develops into a larva, and the larva develops into a polyp. There are many variations on this general life cycle. Obviously, species that exist only as polyps or medusae have a life cycle without the other form.

cnidarian ecology

Cnidarians can be found in almost all ocean habitats. A few species live in fresh water. Jellyfish spend most of their lives as medusae. They live virtually everywhere in the ocean. They prey on zooplankton, other invertebrates, and the eggs and larvae of fish. Corals form large colonies in shallow tropical water. They are confined to shallow water because they have a symbiotic relationship with algae that live inside of them. The algae need sunlight for photosynthesis, so they must stay relatively close to the surface of the water to get enough light. Corals exist only as polyps. They catch plankton with their tentacles. Many corals form a hard, mineral exoskeleton. Over time, this builds up to become a coral reef. A coral reef is pictured in Figure 12.10. Coral reefs provide food and shelter to many other ocean organisms. Watch this beautiful National Geographic video to learn more about corals, coral reefs, and coral reef life: http://video MEDIA Click image to the left or use the URL below. URL:

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cnidarians

Cnidarians are invertebrates such as jellyfish and corals. They belong to Phylum Cnidaria. All cnidarians are aquatic. Most of them live in the ocean. Cnidarians are a little more complex than sponges. Besides specialized cells, they have tissues and radial symmetry. There are more than 10,000 cnidarian species, see Figure 12.6.

sponges and other organisms

Many sponges live on coral reefs, like the one in Figure 12.5. Reef sponges typically have symbiotic relationships with other reef species. For example, the sponges provide shelter for algae, shrimp, and crabs. In return, they get nutrients from the metabolism of the organisms they shelter. Sponges are a source of food for many species of fish. Because sponges are anchored to a reef or rock, they cant run away from predators. However, their sharp spicules provide some defense. They also produce toxins that may poison predators that try to eat them.

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how sponges reproduce

Sponges reproduce both asexually and sexually. Asexual reproduction occurs by budding. Sexual reproduction occurs by the production of eggs and sperm. Males release sperm into the water through the osculum. Sperm may enter a female sponge through a pore and fertilize her eggs. The resulting zygotes develop into larvae. Unlike sponge adults, sponge larvae can swim. They have cilia that propel them through the water. As larvae develop and grow, they become more similar to an adult sponge and lose their ability to swim.

specialized cells in sponges

Sponges have several different types of specialized cells, although they lack tissues. You can see the basic sponge body plan and specialized cells in Figure 12.4. Some of the specialized cells grow short, sharp projections called spicules. Spicules make up the sponges internal skeleton, or endoskeleton. The endoskeleton helps to support and protect the sponge. Other specialized cells are involved in feeding. Sponges are filter feeders. They filter food out of the water as it flows through them. Sponges pump water into their body through specialized pore cells called porocytes. The water flows through a large central cavity. As it flows by, specialized cells called collar cells trap and digest food particles in the water. Specialized cells called amebocytes carry nutrients from the digested food to the rest of the cells in the sponge. As water flows through the sponge, oxygen diffuses from the water to the sponges cells. The cells also expel wastes into the water. The water then flows out of the sponge through an opening called the osculum.

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sponges

Sponges are aquatic invertebrates that make up Phylum Porifera. The word porifera means pore-bearing. As you can see from the close-up view in Figure 12.3, a sponge has a porous body with many small holes in it. There are at least 5000 living species of sponges. Almost all of them inhabit the ocean. Most live on coral reefs or the ocean floor. Adult sponges are unable to move from place to place. They have root-like projections that anchor them to surfaces. They may live in colonies of many sponges. You can visit the incredible world of sponges by watching this short video: . MEDIA Click image to the left or use the URL below. URL:

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medusa and polyp

Cnidarians have two basic body forms, called medusa and polyp: The medusa (medusae, plural) is a bell-shaped form. It is typically able to move. The polyp is a tubular form. It is usually attached to a surface and unable to move. As you can see in Figure 12.8, both body plans have radial symmetry. Some cnidarian species alternate between medusa and polyp forms. Other species exist in just one form or the other.

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special features of cnidarians

An interesting feature of all cnidarians is one or more stingers called nematocysts. You can see a nematocyst in the sketch of a hydra in Figure 12.7. The nematocyst is long and thin and has a poison barb on the end. When not in use, it lies coiled inside a special cell. Nematocysts are used to attack prey or defend against predators. Watch this awesome animation to see a nematocyst in action: http://commons.wikimedia.org/wiki/File:Nematocyst.gif Another interesting feature of many cnidarians is the ability to produce light. The production of light by living things is called bioluminescence. A more familiar example of bioluminescence is the light produced by fireflies. In cnidarians, bioluminescence may be used to startle predators or to attract prey or mates. Watch this short video to see an amazing light show put on by a jellyfish at the Monterey Aquarium in Monterey, California: MEDIA Click image to the left or use the URL below. URL:

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instructional diagrams

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questions

Adult sponges

a. are able to swim

b. have specialized tissues

c. are all predators

-->  d. may produce toxins for defense

___name of the phylum to which sponges belong

a. porocyte

b. spicule

c. coral reef

-->  d. Porifera

e. osculum

f. Cnidaria

g. amebocyte

___opening through which water flows out of a sponge

a. porocyte

b. spicule

c. coral reef

d. Porifera

-->  e. osculum

f. Cnidaria

g. amebocyte

Which statement about reproduction in sponges is false?

a. Sponges reproduce both asexually and sexually

b. Sponges reproduce asexually by budding

-->  c. Male sponges release sperm through amebocytes

d. Sperm may enter a female sponge through a pore

What is the function of collar cells in a sponge?

a. They hold the head upright

-->  b. They trap and digest food

c. They expel wastes

d. They produce gametes

___specialized cell through which water enters the body of a sponge

-->  a. porocyte

b. spicule

c. coral reef

d. Porifera

e. osculum

f. Cnidaria

g. amebocyte

___name of the phylum to which jellyfish belong

a. porocyte

b. spicule

c. coral reef

d. Porifera

e. osculum

-->  f. Cnidaria

g. amebocyte

All cnidarians

-->  a. are aquatic

b. are less complex than sponges

c. have bilateral symmetry

d. have a complete digestive system

___one of many short, sharp projections that make up a sponges endoskeleton

a. porocyte

-->  b. spicule

c. coral reef

d. Porifera

e. osculum

f. Cnidaria

g. amebocyte

The polyp form of a cnidarian

-->  a. has a tubular body

b. can swim freely

c. reproduces sexually

d. always changes into a medusa

___structure that builds up over time from the exoskeletons of corals

a. porocyte

b. spicule

-->  c. coral reef

d. Porifera

e. osculum

f. Cnidaria

g. amebocyte

___specialized cell that carries nutrients from digested food to the rest of the cells of a sponge

a. porocyte

b. spicule

c. coral reef

d. Porifera

e. osculum

f. Cnidaria

-->  g. amebocyte

_Almost all sponges live in fresh water.

a. true

-->  b. false

_Adult sponges may live in colonies of many sponges.

-->  a. true

b. false

_Collar cells in a sponge pump water out of the sponges body.

a. true

-->  b. false

_Jellyfish are filter feeders.

a. true

-->  b. false

There are more species of sponges than cnidarians.

a. true

-->  b. false

Most sponges live on coral reefs or the ocean floor.

-->  a. true

b. false

_Sponges are less complex than cnidarians.

-->  a. true

b. false

Sponge larvae are just like adult sponges except smaller.

a. true

-->  b. false

_All cnidarians alternate between polyp and medusa forms.

a. true

-->  b. false

_Cnidarians have an incomplete digestive system with a single opening.

-->  a. true

b. false

Many cnidarians have the ability to produce light.

-->  a. true

b. false

Corals are found at all levels of the ocean, from very deep to shallow water.

a. true

-->  b. false

Sponges have specialized

-->  a. cells

b. tissues

c. organs

d. all of the above

A male sponge releases sperm into the water through a(n)

a. porocyte

-->  b. osculum

c. spicule

d. collar cell

Reef sponges have symbiotic relationships with other species for which they provide

-->  a. shelter

b. nutrients

c. food

d. water

Examples of cnidarians include

a. anemones

b. hydras

c. corals

-->  d. all of the above

Which is function of a nematocyst?

-->  a. attacking prey

b. producing gametes

c. attaching to reefs

d. filtering food

The medusa form of a cnidarian

-->  a. reproduces sexually

b. is unable to move

c. has a brain

d. all of the above

The larval form of a sponge

a. has a nerve net

-->  b. has cilia

c. cannot move

d. produces food

diagram questions

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