1
Simple Invertebrates
After you read this section, you should be able to answer these questions:
•
What are the characteristics of invertebrates?•
What are the four groups of simple invertebrates?What Are Invertebrates?
About 96% of all animal species are invertebrates. An
invertebrate is an animal that does not have a backbone. Worms, jellyfish, octopuses, and butterflies are some exam-ples of invertebrates. More than 1 million invertebrates have already been named. Scientists think there are millions of invertebrates that have not been found or named.
BODY PLANS
Invertebrates have three basic body plans: bilateral, radial, or asymmetrical. Most animals have bilateral sym-metry. The three basic body plans are decribed below.
Animal Body Plans
Radial Symmetry Bilateral Symmetry
This ant has bilateral symmetry. The two sides of its body mirror each other. On each side of its body, the ant has one eye, one antenna, and three legs.
This sea star has radial symmetry. Its body is organized around the center, like spokes on a wheel.
Asymmetry
This sponge is asymmetrical. You cannot draw a straight line to divide its body into equal parts. Its body is not organized around a center.
BEFORE YOU READ National Science Education Standards LS 1d, 1f, 2a, 3a, 3c, 5a
STUDY TIP
Outline As you read, make
an outline of this section. Use the header questions to help you make your outline.
TAKE A LOOK
1. Describe What is bilateral
symmetry?
2. Apply Concepts What
NEURONS AND GANGLIA
All animals, except sponges, have special tissues that allow them to sense their environment. These tissues make fibers called neurons. Neurons carry messages around the body to control the animal’s actions. Simple invertebrates have nerve cords. Nerve cords are bundles of neurons that carry messages along a single path.
Some invertebrates have nerve cells called ganglia (singular, ganglion). A ganglion is a mass of nerve cells grouped together. Each ganglion controls a different part of the body. The ganglia are connected by nerve cords. In complex invertebrates, a brain controls the ganglia and nerves throughout the body.
GUTS
Most animals digest food in a gut. A gut is a hollow space lined with cells. The cells make chemicals to break down food into small particles. The cells then absorb the food particles.
In complex animals, the gut is inside a coelom. A
coelom is a body cavity that surrounds the gut. The coelum keeps other organs, such as the heart, separated from the gut. That way, movements in the gut do not disturb the actions of other organs. In simple invertebrates, the gut is not surrounded by a coelom.
Gut
Coelom Earthworms have a coelom fi lled with fl uid. The gut is inside the coelom.
What Are Sponges?
Sponges are the simplest invertebrates. They are asym-metrical. They have no tissues, gut, or neurons. Adult sponges may move only a few millimeters each day or not all.
Scientists once thought sponges were plants. However, READING CHECK
3. Identify In an
inverte-brate’s body, what connects all the ganglia?
TAKE A LOOK
4. Identify What is the
Water carries food into the sponge through pores. Inside the sponge, collar cells remove food from the water. The water exits through an osculum.
Water fl ow
Collar cells
line the central cavity of a sponge.
Pore cells have
holes that let water fl ow into the sponge. Osculum
Pores
HOW SPONGES FEED
As shown above, a sponge has a special way of get-ting food. Water flows into the body of a sponge through pores. Pores are holes on the outside of a sponge. The water flows into a cavity in the middle of the sponge.
Collar cells line this cavity. Collar cells take in tiny plants and animals from the water and digest them. Water leaves through a hole at the top of the sponge. This is called the
osculum.
REGENERATION
Sponges have some unusual abilities. If a sponge is forced through a strainer, the cells can come back together to re-form a new sponge. If part of a sponge is broken off, the missing part can grow back. This ability is called regeneration. A part of the sponge that is broken off can also grow into a new sponge. This means sponges can use regeneration as a way to reproduce.
SUPPORT IN A SPONGE BODY
Most sponges have skeletons that support their bod-ies and protect them from predators. This skeleton is not made of bones, however. It is made of small, hard fibers called spicules. Some spicules are straight, some are curved, and some are star-shaped.
TAKE A LOOK
5. Identify What is the
function of collar cells?
6. Identify Through what
structure does water leave a sponge?
STANDARDS CHECK
LS 2a Reproduction is a
char-acteristic of all living systems; because no living organism lives forever, reproduction is essential to the continuation of every spe-cies. Some organisms reproduce asexually. Others reproduce sexually.
7. Apply Concepts When
What Are Cnidarians?
If you have ever been stung by a jellyfish, then you have touched a cnidarian. Jellyfish are members of a group of invertebrates that have stinging cells. Animals in this group are called cnidarians.
Cnidarians have complex tissues and a gut for digest-ing food. They also have a system of nerve cells within their bodies. When some cnidarians are broken into pieces, the pieces can become new cnidarians.
TWO BODY FORMS
A cnidarian body can have one of two forms—medusa or polyp. The medusa and the polyp forms are shown below. Medusas swim through the water. Polyps usually attach to a surface. Some cnidarians change forms during their lives. However, many cnidarians are polyps for their whole lives.
Medusa
Polyp
Both the medusa and the polyp forms of a jellyfi sh have radial symmetry
STINGING CELLS
All cnidarians have tentacles covered with stinging cells. The touch from another organism activates these stinging cells. Each stinging cell shoots a tiny spear into the organism. Each spear carries poison. Cnidarians use stinging cells to protect themselves and to catch food.
A tiny barbed spear is coiled inside each stinging cell.
When the tiny spear is fi red, the long barbed strand is shot into the prey. READING CHECK
8. Identify What do all
cnidarians have in common?
TAKE A LOOK
9. Identify What kind of
symmetry do jellyfi sh have?
TAKE A LOOK
10. List Name two reasons
KINDS OF CNIDARIANS
There are three classes of cnidarians: hydrozoans, jellyfish, and sea anemones and corals. The figure below shows each type of cnidarian.
Hydrozoan
Sea anemone Jellyfi sh
Coral
Hydrozoans live in both freshwater and saltwater. Most hydrozoans live as polyps their entire lives. Jellyfish use their tentacles to catch fish and other invertebrates. They spend most of their lives as medusas. Sea anemones and corals spend their lives as polyps. Corals can build large and colorful reefs.
What Are Flatworms?
When you think of worms, you probably think of earth-worms. However, there are many other kinds of earth-worms. Many of them are too small to see without a microscope. The simplest worms are the flatworms.
Flatworms are divided into three classes: planarians, flukes, and tapeworms. All flatworms have bilateral sym-metry. Most flatworms have a head and two eyespots. Some flatworms have a bump on each side of their heads. These are called sensory lobes. The flatworm uses sensory lobes to find food.
TAKE A LOOK
11. Apply Concepts Which
of the cnidarians in this fi gure have the polyp body form?
READING CHECK
12. Identify What are the
PLANARIANS
Planarians are flatworms that live in fresh water or damp places. As shown below, they have a head, eyespots, and sensory lobes. Most planarians are predators. They eat other animals or parts of animals and digest the food in a gut. Planarians use their sensory lobes to find food. Planarians also have a nervous system with a brain.
Eyespot Sensorylobe
FLUKES
Flukes are parasites. A parasite is an organism that feeds on another living thing. This organism is called the host.
Most flukes live and reproduce inside a host animal. The fertilized eggs of a fluke pass out of the animal’s body with its waste products. If these eggs infect water or food, animals may eat them. These eggs would then grow into new flukes inside these animals. Flukes have no eyespots or sensory lobes. They use suckers to attach to their hosts.
Flukes use suckers to attach to their hosts. Most fl ukes are just a few millimeters long.
TAPEWORMS
Tapeworms are also parasites. Like flukes, tapeworms live, feed, and reproduce inside a host. They have no eyespots, sensory lobes, or gut. Tapeworms simply attach to the intestines of their hosts and absorb nutrients. READING CHECK
13. Explain What do
planarians use their sensory lobe for?
TAKE A LOOK
14. Identify What structures
do fl ukes use to attach to a host?
Tapeworms can grow very long. Some are longer than a bus!
What Are Roundworms?
Roundworms have bodies that are long, slim, and round, like short pieces of spaghetti. Like other worms, they have bilateral symmetry. Roundworms have a simple nervous system and brain.
Most species of roundworms are very small. A single rotten apple could contain 100,000 roundworms! These tiny worms get nutrients by breaking down the tissues of dead plants and animals. This process helps put some nutrients back into the soil.
Some roundworms are parasites. Several roundworms, such as pinworms and hookworms, can infect humans. Humans can become infected with roundworms by eating meat that has not been fully cooked. In humans, roundworms can cause fever, tiredness, and digestive problems.
This hookworm is a tiny larva. Even as an adult, it will be less than 15 mm.
Critical Thinking
15. Compare How does thesize of a tapeworm compare to the size of a planarian?
READING CHECK
16. Describe Describe the
SECTION VOCABULARY
coelom a body cavity that contains the internal
organs
ganglion a mass of nerve cells
gut the digestive tract
invertebrate an animal that does not have a
backbone 1. Identify What do all invertebrates have in common?
2. List What are the four groups of simple invertebrates?
3. Explain How does the gut of a complex animal differ from that of a simple animal?
4. List Give two ways sponges use regeneration.
5. Identify What are the two cnidarian body forms?
6. Infer Why do you think it would be important to a parasite that its host survive?
7. Compare Complete the chart below to compare the different types of flatworms.
Type of fl atworm Parasitic or
nonparasitic?
Features
Planarians head, eyespots,
sensory lobes, brain
Chapter 15 Invertebrates
SECTION 1 SIMPLE INVERTEBRATES1. The two sides of an animal’s body mirror
each other.
2. bilateral 3. a nerve cord 4. coelom
5. to remove food from water 6. osculum
7. Asexual—only one parent produces offspring. 8. stinging cells
9. radial
10. to protect themselves, to catch food 11. hydrozoan, coral, sea anemone 12. flatworms
13. to find food 14. suckers
15. Some tapeworms can grow longer than a
bus. Planarians are often only 15 mm long.
16. It is long, slim, and round like a short piece
of spaghetti.
Review
1. They do not have backbones.
2. sponges, cnidarians, flatworms, roundworms 3. The gut of a complex animal is surrounded
by a coelom.
4. to grow back missing parts, to reproduce 5. medusa and polyp
6. A parasite feeds on its host. If the host dies,
the parasite won’t have a way to get nutri-ents, and it will die as well.
7. Type of
fl atworm Parasitic or nonparasitic? Features
Planarians nonparasitic head, eyespots, sensory lobes, brain Flukes parasitic suckers, no eyespots,
no sensory lobes Tapeworms parasitic no eyespots, no sensory lobes, no gut
8. No, some roundworms are parasites. Others
eat dead tissues of plants and animals.
SECTION 2 MOLLUSKS AND ANNELID WORMS
1. gastropods
2. to scrape food from surfaces
4. closed 5. squid
6. protects it from predators, keeps land
mol-lusks from drying out
7. earthworms, marine worms, leeches
8. the wastes earthworms leave after breaking
down plant and animal matter
Review
1. In an open circulatory system, blood is
pumped into sinuses in the animal’s body. In a closed circulatory system, blood is pumped through a network of blood vessels that form a closed loop.
2. foot, visceral mass, mantle, shell
3. Gastropods and bivalves are the other two
classes of mollusks; squids and octopuses are two types of cephalopods; clams and oysters are two types of bivalves.
4. Their castings make soil richer. The tunnels
they dig let water and air reach deep into the soil.
5. No, only some leeches are parasites. Some
feed on dead animals, and some eat insects, slugs, and snails.
SECTION 3 ARTHROPODS
1. 75%
2. head, thorax, and abdomen 3. protein and chitin
4. compound
5. 188 segments, 177 segments
6. Crustaceans have two pairs of antennae. 7. cephalothorax and abdomen
8. chelicerae
9. egg, larva, pupa, adult
10. Complete metamorphosis has four stages,
and incomplete has three. Incomplete meta-morphosis has a nymph stage. The nymphs look like small adults.
Review
1. segmented body with specialized parts,
exo-skeleton, well-developed nervous system, jointed limbs
2. Arachnids have two main body parts, and most