Invertebrate Characteristics
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: September 17, 2012
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Invertebrate Characteristics
1 Invertebrate Characteristics
• Describe general characteristics of invertebrates.
What type of animal is a Sea Cucumber?
That is a Sea Cucumber, and it is an animal. The Sea Cucumber is a soft-bodied invertebrate that is related to the
starfish and sea urchins. There are over 1,100 species that are found worldwide in intertidal zones as well as deep
waters.
Characteristics of Invertebrates
The majority of animals today are invertebrates. They have a wide range of physical traits and ways of life. Modern
invertebrates include animals as different as the sponge and tarantula. Why are both of these animals classified as
invertebrates? What traits do they have common?
One trait invertebrates like the sponge and tarantula share is lack of a backbone. In fact, they don#8217;t have any
bones at all. These are defining traits of all invertebrates. Some invertebrates have a skeleton, but it isn#8217;t made
of bone. Many other traits of invertebrates show considerable diversity.
Digestion
Invertebrates have one of two types of digestive system: an incomplete or complete digestive system. Both are shown
in Figure 1.2. An incomplete digestive system consists of a digestive cavity with one opening. The single opening
serves as both mouth and anus. A complete digestive system consists of a digestive tract with two openings. One
opening is the mouth. The other is the anus.
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FIGURE 1.1
Examples of Invertebrates. Both a sponge (left) and tarantula (right) are invertebrates. Can you identify any traits
they share?
FIGURE 1.2
Two Types of Digestive Systems in Invertebrates. On the left is an incomplete digestive system, found in a jellyfish;
on the right is the complete digestive system of a roundworm. Invertebrates may have either of these two types
of digestive system. Find the parts of each digestive system in each drawing. How do the two systems differ?
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Concept 1. Invertebrate Characteristics
Movement
All invertebrates can move on their own during at least some stage of their life cycle. However, they may differ in
how they move. Several ways are described below.
• Some invertebrates are simply carried along by water currents. They cannot control their movement in a
particular direction. An example is a jellyfish.
• Other invertebrates can contract muscles to move independently of water currents or on solid surfaces. They
can also control the direction in which they move. An example is a roundworm. It can move forward and to
the left or right.
• Still other invertebrates have specialized appendages for movement. For example, they may have jointed legs
for walking or climbing or wings for flying. An example is an insect such as a fly.
Nervous System
Most invertebrates have a nervous system. The nervous system allows them to sense and respond to their environment. The simplest invertebrate nervous system is just a network of nerves that can sense touch, called a nerve
net (see Figure 1.3). Most invertebrates have a more complex nervous system. It may include a brain and several
different sense organs.
FIGURE 1.3
Nerve Net (Coral). The body of a coral
is lined with a net of nerves that can
detect touch. How might this ability be
adaptive?
Reproduction
Most invertebrates reproduce sexually. Diploid adults produce haploid gametes (sperm and eggs). In some species,
the same individuals produce both sperm and eggs. In other species, sperm and eggs are produced by separate male
and female individuals. Fertilization occurs when a sperm and an egg fuse to form a diploid zygote. The zygote
develops into an embryo and eventually into a new adult organism. On the way, it may pass through one or more
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larval stages. A larva (plural, larvae) is a juvenile, or immature, stage of an animal. It is generally quite different in
form and function from the adult form of the species. For example, the larva may be able to swim freely, whereas
the adult must remain permanently attached to a solid surface.
Some invertebrates can also reproduce asexually. This may occur by fission or budding. Fission takes place when
an animal simply divides into two parts. Each part then regrows the missing part. The result is two whole organisms.
Budding may take place when a parent forms a small bump, or bud. The bud remains attached to the parent while it
develops into a new individual.
Vocabulary
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appendage: Limbs, such as legs or wings.
brain: Central nervous system organ inside the skull; the control center of the nervous system.
budding: Type of asexual reproduction in yeasts in which an offspring cell pinches off from the parent cell.
complete digestive system: Digestive system consisting of a digestive tract and two body openings (mouth
and anus).
fission: Type of asexual reproduction that occurs when an animal simply divides into two parts.
incomplete digestive system: Digestive system that consists of a digestive cavity and a single opening that
serves as both mouth and anus.
invertebrate: Animal that lacks a vertebral column, or backbone.
larva (plural, larvae): Juvenile stage that occurs in the life cycle of many invertebrates, fish, and amphibians
and that differs in form and function from the adult stage.
nerve net: The simplest invertebrate nervous system; a network of nerves that can sense touch.
nervous system: Organ system that carries electrical messages throughout the body.
Summary
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The majority of living animals are invertebrates. Invertebrates lack a backbone.
Invertebrates may have an incomplete or a complete digestive system.
Invertebrates vary in how they move and in the complexity of their nervous system.
Most invertebrates reproduce sexually. After hatching, many invertebrates pass through one or more larval
stages that are different from the adult stage.
Practice
Use these resources to answer the questions that follow.
• http://www.hippocampus.org/Biology → Non-Majors Biology → Search: Asymmetrical Invertebrates
1. Which is the only group of animals to lack symmetry?
2. Describe the wall of a sponge.
3. What are spicules?
• http://www.hippocampus.org/Biology → Non-Majors Biology → Search: Radially Symmetric Invertebrates
1. What is meant by #34;radially symmetric#34;?
2. What is a nematocyst?
3. Compare Cnidaria to Ctenophora.
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Concept 1. Invertebrate Characteristics
• http://www.hippocampus.org/Biology → Non-Majors Biology → Search: Bilaterally Symmetric Invertebrates
1. What is meant by #34;bilaterally symmetric#34;?
2. Define triploblastic.
3. Describe three examples of bilaterally symmetric invertebrates.
Review
1. Describe the range of variation in the nervous systems of invertebrates.
2. Create a diagram to show the life cycle of an invertebrate with a larval stage. Include simple sketches of the adult
and larval stages of the animal.
3. Compare and contrast incomplete and complete digestive systems. Why is a complete digestive system more
efficient?
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