Download Study Notes for Sponges and Cnidarians

Study Notes for Sponges and Cnidarians
SPONGES
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The least complex in body structure of the animal group are sponges, cnidarian, flatworms,
and roundworms.
Characteristics of Sponges
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All sponges live in water.
Most are found in warm, shallow salt water near the coast, although some are found at ocean
depths of 8500m or more.
A few species live in freshwater rivers, lakes, and streams.
Sponges grow in many shapes, sizes and colors.
Some have radial symmetry, but most are asymmetrical.
Adult sponges live attached to one place. They are often found with other sponges in colonies
that never move, unless they are washed away by a strong wave.
Organisms such as sponges that remain attached to one place during their lifetimes are
sessile.
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The body of a sponge is covered with many small openings called pores. It is from the pores
that sponges get their phylum name, Porifera. Porifera comes from a Latin word meaning “pore
bearing”.
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A sponge’s body is a hollow tube closed at the bottom but with an opening at the top. The body
wall has two cell layers made up of several different types of cells. There are cells that help a
sponge get food, cells that digest food, cells that carry nutrients to all parts of the sponge, and
cells that enable water to flow into the sponge.
Obtaining Food
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Sponges filter food from water as it is pulled in through their pores.
Bacteria, algae, protozoans, and other materials are filtered out of the water to be used as
food.
Organisms that obtain food in this way are called filter feeders.
Cells that line the inside of the sponge, called collar cells, help water move through the
sponge.
Collar cells have flagella.
The beating of the flagella in these cells moves water through the sponge, bringing oxygen to
the cells and carrying away wastes.
The bodies of many sponges contain sharp, pointed structures called spicules. The softbodied sponges people use to take baths or to wash their cars have a skeleton of a fibrous
material called sponging.
Other sponges are supported by both spicules and sponging.
Scientists classify sponges based on the kinds of materials that make up their skeletons.
Reproduction
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Sponges can reproduce both sexually and asexually.
They can reproduce by forming buds.
New sponges can also form from small pieces that break off the parent sponge.
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Sponge growers cut sponges into pieces, attach weights to them, and put them back into the
ocean so the sponges can regenerate.
Regeneration is the ability of an organism to replace body parts.
Sponges can reproduce sexually by egg and sperm.
Sperm are released in the water and carried by currents to other sponges where they fertilize
the eggs.
The fertilized egg develops into a young organism called a larva (plural, larvae).
Larvae usually look very different from adults. Sponge larvae have cilia that allow them to
swim about in the water. After a short time, the larvae settle down on objects where they will
remain and grow into adult sponges.
Origin of Sponges
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Sponges appeared on Earth about 600 million years ago in the Cambrian period.
Their collar cells are similar to a type of colonial protozoan that is thought to be the ancestor of
sponges. No other animal species is known to have evolved from sponges.
CNIDARIANS
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The word cnidarian refers to the stinging cells that all members in the phylum have. The word
cnidaria is Latin for “stinging cells”.
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There are many different groups of cnidarians: the hydra group, the jellyfish, and the corals
and sea anemones.
Although many types of hydras live in fresh water, most cnidarians live in salt water.
Most jellyfish live as individual organisms, but hydras and corals tend to form colonies. Corals,
for example, live in colonies of polyps. Each polyp secretes a calcium carbonate shelter
around its body. Over time, these calcium carbonate shelters join to those of neighboring
corals to form a coral reef.
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All cnidarians are radially symmetrical, with more complex bodies than those of sponges.
They have two cell layers that are arranged into tissues and a digestive cavity where food is
broken down. Most cnidarians have armlike structures called tentacles that surround the
mouth. The tentacles are armed with stinging cells that help the organism capture food.
Two Body Plans
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Cnidarians have two body plans.
The polyp is shaped like a vase and is usually sessile.
The medusa is bell-shaped and free-swimming.
They hydra us an example of a polyp form, whereas the jellyfish is an example of
a medusa form.
 Some cnidarians go through both polyp and medusa stages during their life
cycles.
 Cnidarians have a system of nerve cells called a nerve net. The nerve net
carries impulses and connects all parts of the organism.
Reproduction
 Cnidarians reproduce both asexually and sexually. Polyps reproduce asexually
by producing buds that eventually fall off the parent and develop into new polyps.
 Polyps can also reproduce sexually by producing eggs or sperm. Sperm are
released into the water and fertilize the eggs.
 Medusa forms of cnidarians have both an asexual stage of reproduction and a
sexual one. Free-swimming medusae produce eggs and sperm and release
them into the water. The eggs are fertilized and develop into larvae. The larvae
eventually settle down and grow into polyps. Young medusae bud off the polyp,
and the cycle begins again.
ORIGIN OF CNIDARIANS
 Cnidarians were present on Earth during the Precambrian era more than 600
million years ago.
 Scientists think that the first form of cnidarian was the medusa. Polyps may have
formed from larvae of medusae that became permanently attached to a surface.
 Most of the cnidarian fossils are fossils of corals.
Cnidarians and Coral Reefs
 Many of the world’s shallow seas are dominated by cnidarians. Coral reefs are
found in warm waters of tropical seas between the latitudes of 30 degrees south
and 30 degrees north. Besides corals, other cnidarians are found on these reefs.
Sea anemones often attach themselves to dead coral, and many other
organisms live on or near coral reefs.
 The largest reef system in the world is the Great Barrier Reef in Australia