CHAPTER 9
LESSON 2
Introduction to Animals
Invertebrates
Key Concepts
What do you think? Read the two statements below and decide
whether you agree or disagree with them. Place an A in the Before column
if you agree with the statement or a D if you disagree. After you’ve read
this lesson, reread the statements to see if you have changed your mind.
Before
Statement
• What characteristics do
invertebrates have in
common?
• How do the groups of
invertebrates differ?
After
3. A sponge is not an animal because it cannot
move.
4. There are more arthropods on Earth than all
other kinds of animals combined.
3TUDY#OACH
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What is an invertebrate?
Have you ever seen sea anemones? Perhaps you saw them
at an aquarium. Many people think sea anemones look like
colorful flowers. But sea anemones are not flowers. They are
animals. They trap food in their fingerlike tentacles.
Anemones do not have backbones.
Recall how animals support their bodies. Most animals
with an endoskeleton have a backbone for support. These
animals are called vertebrates. Animals without backbones
are called invertebrates. The bodies of most invertebrates are
supported by either a hydrostatic skeleton—a fluid-filled
internal cavity—or an exoskeleton—a hard outer covering.
Some invertebrates have endoskeletons.
Invertebrates make up about 95 percent of all known
animal species. In this lesson, you will read about eight of
the most common invertebrate phyla. Recall that phyla are
one of the levels of taxons.
Invertebrates have many adaptations for survival. Some
invertebrates are parasites. Parasites are animals that survive by
living inside or on another organism, get food from the organism, and
do not help in the organism’s survival. Other invertebrates hunt
their food. Some invertebrates can change the color of their
skin to match their environments.
Reading Essentials
Make an Outline As you
read, summarize the
information in the lesson by
making an outline. Use the
main headings in the lesson
as the main headings in your
outline. Use your outline to
review the lesson.
Key Concept Check
1. Examine What
characteristics do
invertebrates have in
common?
Introduction to Animals
147
Sponges
ACADEMIC VOCABULARY
attach
(verb) to fasten
Visual Check
2. Explain How do
sponges obtain food?
The oldest branch of the animal family tree, phylum
Porifera (puh RIH fuh ruh), includes the sponges. Sponges
are often called simple animals because they have only a few
types of cells and no true tissues. Sponges live in water.
Adult sponges cannot move. They attach to rocks and other
underwater structures. Sponges
Sponge
take in food when water passes
through their bodies, as shown
Water out
in the figure to the right. Special
cells inside the sponge filter out
food particles in the water.
Sponges have tiny, stiff fibers
that support their bodies.
Scientists group sponges by the
Water in
kinds of materials that make up
these fibers. The most common
group of sponges has fibers made
of either silica or the protein
spongin, or both. These sponges
are harvested and sold as natural sponges. The stiff fibers
make the sponges useful for scrubbing, but the fibers can
scratch shiny surfaces.
Cnidarians
3. Identify Circle the part
of the nematocyst that
injects the poison.
Cnidarians are more complex than sponges because
cnidarians have true tissues. Some cnidarians, such as corals
and anemones, remain attached to underwater surfaces for
all of their adult lives. Others, such as jellyfish, can swim.
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Introduction to Animals
Reading Essentials
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Visual Check
Corals, anemones, jellyfish, hydras, and Portuguese
man-of-wars are all members of phylum Cnidaria
(nih DAYR ee uh). The
Nematocyst
name Cnidaria comes from
special cells these animals
use to catch their prey.
These cells—nematocysts
(NE mah toh sihsts)—are
shown in the figure to the
right. Nematocysts can
inject poison into animals
that come in contact with
them. Cnidarians have
radial symmetry.
Flatworms
The common name for an animal in the phylum
Platyhelminthes (pla tih hel MIHN theez) is flatworm. This
name describes it accurately because its body shape is flat.
Flatworms have bilateral symmetry; each worm has a left
side and a right side that are similar.
Most flatworms live in water, either freshwater or salt
water. Some flatworms are free-living. The planarian swims
freely in water and ingests food through a tube on the
underside of its body. Other flatworms are parasites. The
liver fluke is a parasite and can infect humans.
Segmented Worms
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Have you ever held an earthworm? Did you notice that
its body was like a tube of tiny rings? The name for the
phylum that includes earthworms, Annelida (ah NEL ud uh),
means “little rings.” These rings, as shown in the figure
below, are called segments. Each segment is a fluid-filled
compartment. Therefore, a segmented worm has a hydrostatic
skeleton. Segmented worms have bilateral symmetry.
Did you also notice that the sides of the earthworm’s
body felt prickly? The prickles are tiny, stiff hairs called setae
(SEE tee). Setae help earthworms grip surfaces. As earthworms
tunnel through soil, they take the soil into their bodies and
absorb nutrients from it. Their tunnels help break up soil.
Segmented worms also can be parasites. Leeches attach their
mouths to other animals and suck blood.
Reading Check
4. Explain Why are
Annelida called segmented
worms?
Visual Check
5. Draw In the cross-section
of the earthworm, draw a
line to show the earthworm’s
bilateral symmetry.
Earthworm
Fluid
Reading Essentials
Introduction to Animals
149
Mollusks
Make a horizontal two-tab
book to identify similarities
and differences in
invertebrates.
Invertebrates
Common
Characteristics Differences
Reading Check
6. Identify Name one
type of mollusk that does
not have a shell.
On summer mornings you might notice thin, slimy trails
across a sidewalk. These trails likely were made during the
night by snails or slugs searching for food. Snails and slugs
are mollusks in phylum Mollusca (mah LUS kuh).
Most mollusks have a footlike muscle that the animal
generally uses for movement. A mollusk also has a mass of
tissue called a mantle. A mantle is a thin layer of tissue that covers
a mollusk’s internal organs. The mantle also is involved in
making the shell of most mollusks.
A shell supports and protects a mollusk’s soft body. Most
mollusks, such as snails, have external shells. Some mollusks,
such as slugs, do not have shells. Other mollusks, such as
squids and octopuses, have internal shells. Mollusks have
bilateral symmetry.
Mollusks obtain food in different ways. Some mollusks,
such as clams, oysters, and scallops, filter food particles from
the water in which they live. Other mollusks, such as
octopuses and squids, are predators and catch their prey in
long, strong tentacles.
Roundworms
Other roundworms are harmless to plants and humans.
The vinegar eel is a roundworm that feeds on organisms
used in making vinegar. Vinegar eels are harmless to
humans, but they are removed from vinegar by the
manufacturer before the vinegar can be sold.
Reading Check
7. Explain Why must a
roundworm molt?
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Introduction to Animals
All roundworms have bilateral symmetry. Roundworms
have a hydrostatic skeleton for movement. A roundworm
has a hard outer covering, called a cuticle, which protects its
body. The cuticle does not grow as the roundworm grows.
When a roundworm grows too large for its cuticle, it sheds
its cuticle and replaces it with a larger cuticle. An outer
covering is shed and replaced in a process called molting.
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Animals in phylum Nematoda (ne muh TOH duh) are
called nematodes or roundworms. Some roundworms are
parasites. Some parasitic roundworms infect plant roots,
while others infect humans. Most roundworms live in soil
and are too small to see without a magnifying lens. These
roundworms eat dead organisms and return nutrients to the
soil. They typically are harmless to humans.
Arthropods
Can you imagine a billion billion of something? Scientists
estimate that is how many individual arthropods exist on
Earth. There are more animals in phylum Arthropoda
(ar THRAH puh duh) than in all other animal phyla
combined. Arthropods have bilateral symmetry.
Like a roundworm, an arthropod has a hard outer
covering, so it must molt in order to grow. An arthropod has
an exoskeleton for both movement and protection. Its
muscles attach to the exoskeleton. An arthropod uses its
muscles when moving its jointed appendages. An appendage
is a structure, such as a leg or an arm, that extends from the
central part of the body.
The body of an arthropod has three parts: a head, a thorax,
and an abdomen. The head contains sense organs that see,
feel, and taste the environment. The thorax is the part of the
body where legs attach. The abdomen contains intestines
and reproductive organs.
Arthropods have open circulation. This means their blood
is not in vessels. Instead, it washes over internal organs.
Reading Check
8. Name What are the
three parts of an arthropod’s
body?
Insects Most arthropods are insects. Scientists call them
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
hexapods because they have six legs. Insects are the only
arthropods that have the ability to fly.
Another trait of insects is metamorphosis. In
metamorphosis, the body form of an animal changes as it grows from
an egg to an adult. The stages in the metamorphosis of a
butterfly are egg, larva (caterpillar), pupa, and adult
butterfly.
Other Arthropod Groups There are three other major groups
of arthropods. Spiders and scorpions are one group. They
have eight legs used for walking and grasping.
Reading Check
9. Paraphrase In your
own words, describe
metamorphosis.
Crabs and lobsters make up another group. Members of
this group mostly live in water. They have chewing mouthparts
and three or more pairs of legs. Some lobsters have as many
as 19 pairs of appendages.
Centipedes and millipedes are in another group. They
have the most appendages. Generally, a centipede has one
pair of legs per segment and a millipede has two pairs per
segment. Millipedes eat dead plants, but centipedes are
predators.
Reading Essentials
Reading Check
10. Distinguish Which
group of arthropods has the
most appendages?
Introduction to Animals
151
Echinoderms
Have you ever seen a sea star, a sea cucumber, or a sea
urchin? At first glance, they may appear fuzzy and soft. But
if you touch an echinoderm (ih KI nuh durm), from the
phylum Echinodermata (ih kin uh DUR muh tuh), you will
discover that it feels the opposite of soft. Echinoderm means
“spiny skin.” An echinoderm feels spiny due to the hard
endoskeleton just beneath its thin outer skin.
Reading Check
11. Describe Echinoderms
live in what kind of
environment?
All echinoderms live in salt water. They move slowly with
tiny suction-cuplike feet, called tube feet. Their tube feet are
connected to larger tubes called canals. These canals connect
to a central ring that controls water movement within the
animal. Water moves back and forth through the canals and
tube feet. This movement enables echinoderms to grab onto
or let go of any surface they are moving across. Echinoderms
have bilateral symmetry when they are young and radial
symmetry as adults.
Echinoderms develop differently than the other
invertebrate phyla. Their early development is similar to that
of vertebrates. Because of this, echinoderm embryos often
are used to study human development and early growth
patterns.
Key Concept Check
12. Differentiate How
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Introduction to Animals
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do the groups of invertebrates
differ?
Reading Essentials
Mini Glossary
mantle: a thin layer of tissue that covers a mollusk’s internal
organs
molting: a process by which an outer covering is shed and replaced
parasite: an animal that survives by living inside or on another
metamorphosis: the process of change in an animal’s body
form as it grows from an egg to an adult
organism, gets food from the organism, and does not help in
the organism’s survival
1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains
the process of molting.
2. Identify the phylum and type of symmetry for each invertebrate described in the table.
Some of them have been done for you.
Phylum
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a.
Type of
Symmetry
Features
Examples
asymmetry few cell types; no true tissues; cannot move as adults;
attach to underwater structures; stiff fibers support
body
sponges
b.
use nematocysts to inject poison into their prey;
have true tissues
corals, anemones,
jellyfish
c. Platyhelminthes
flat body shape; most live in water; can be parasites
or free-living
planarian, liver fluke
d.
divided into segments; have hydrostatic skeletons;
some are parasites
earthworms, leeches
move with footlike muscle; have a mantle; most have
shells
snails, slugs, clams,
octopuses
f.
most live in soil and are too small to see without
magnification; most harmless; some can infect
humans or plants; molt exoskeleton
vinegar eel, other
roundworms
g. Arthropoda
phylum contains more animals than all other animal
phyla combined; hard exoskeleton; have head, thorax,
and abdomen; open circulation
insects, spiders, crabs,
centipedes
h.
hard, spiny endoskeleton just beneath skin; live in salt
water; move with suction-cuplike feet; invertebrate
sea star, sea cucumber,
sea urchin
e.
bilateral
What do you think
Reread the statements at the beginning of the
lesson. Fill in the After column with an A if you
agree with the statement or a D if you disagree.
Did you change your mind?
Reading Essentials
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END OF
LESSON
Introduction to Animals
153