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Invertebrate Evolution
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: November 18, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
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Chapter 1. Invertebrate Evolution
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Invertebrate Evolution
Outline major events in invertebrate evolution.
Explain the importance of multicellularity.
Compare ectoderm to endoderm to mesoderm.
Compare radial symmetry to bilateral symmetry.
Define cephalization.
Describe a complete digestive system.
Compare a pseudocoelom to a coelom.
Explain segmentation.
Define notochord.
How many different types of beetles are there?
There are about 350,000 species of beetles spread all over the world. But let’s focus on this one. Look at the detail
on this Rhinoceros beetle. The horns are used in fighting other males during mating season, and for digging. The
body of an adult rhino beetle is covered by a thick exoskeleton. A pair of thick wings lay atop another set of wings
underneath, allowing the rhinoceros beetle to fly. Compare those evolutionary adaptations to a simple sponge, and
the evolutionary significance of invertebrates becomes obvious.
Invertebrate Evolution
Invertebrates evolved several important traits before vertebrates even appeared. These traits are now found in just
about all animals.
Multicellularity
The first animal trait to evolve was multicellularity. This was highly adaptive. Multiple cells could do different jobs.
They could evolve special adaptations that allowed them to do their job really well. However, the first invertebrates
still lacked tissues. Sponges represent the first organism at the multicellular stage of invertebrate evolution.
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Tissues
Living cnidarians, such as jellyfish, represent the next stage of invertebrate evolution. This was the evolution of
tissues. It was the first step in the evolution of organs and organ systems. At first, invertebrates developed tissues
from just two embryonic cell layers. There was an outer cell layer called ectoderm and an inner cell layer called
endoderm. The two cell layers allowed different types of tissues to form.
Radial Symmetry
Another trait that evolved early on was symmetry. To understand symmetry, you need to see an animal that lacks
symmetry. A sponge, like the one in Figure 1.1, lacks symmetry. This means it cannot be divided into two identical
halves. A symmetrical organism, in contrast, can be divided into two identical halves. Both the coral polyp and the
beetle in Figure 1.1 have symmetry.
FIGURE 1.1
Symmetry in Invertebrates. Sponges lack symmetry. Radial symmetry evolved first. This was followed by bilateral
symmetry. How do the two types of symmetry differ?
The coral polyp in Figure 1.1 has radial symmetry. This was the first type of symmetry to evolve. The coral has
a distinct top and bottom but not distinct ends. It can be divided into identical halves like a pie, but not into right
and left halves. Animals with radial symmetry have no sense of directions such as forward and backward or left and
right. This makes controlled movement in these directions impossible.
Cephalization
Flatworms represent the next stage of invertebrate evolution. They evolved cephalization. This is the concentration
of nerve tissue at one end of the body, forming a head region. This is highly adaptive. It allows central control of the
entire organism. Cephalization was first step in the evolution of a brain.
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Chapter 1. Invertebrate Evolution
Bilateral Symmetry
An outcome of cephalization was bilateral symmetry. This is demonstrated by the beetle in Figure 1.1. With
concentrated nerve tissue at the head but not at the tail end, the two ends of the body are distinct from each other.
The animal can be divided down the middle to form identical right and left halves. It allows the animal to tell front
from back and left from right. This is needed for controlled movements in these directions.
Mesoderm
Ancestors of flatworms also evolved mesoderm. This is a third layer of cells between the ectoderm and the endoderm
(see Figure 1.2). Evolution of this new cell layer allowed animals to develop new types of tissues, such as muscle.
FIGURE 1.2
Three Cell Layers in a Flatworm. A flatworm has three cell layers.
Complete Digestive System
Early invertebrates had an incomplete digestive system. There was just one opening for the mouth and anus.
Ancestors of modern roundworms were the first animals to evolve a complete digestive system. With a separate
mouth and anus, food could move through the body in just one direction. This made digestion more efficient. An
animal could keep eating while digesting food and getting rid of waste. Different parts of the digestive tract could
also become specialized for different digestive functions. This led to the evolution of digestive organs.
Pseudocoelom and Coelom
Ancestors of roundworms also evolved a pseudocoelom. This is a partial body cavity that is filled with fluid. It
allows room for internal organs to develop. The fluid also cushions the internal organs. The pressure of the fluid
within the cavity provides stiffness. It gives the body internal support, forming a hydrostatic skeleton. It explains
why roundworms are round and flatworms are flat. Later, a true coelom evolved. This is a fluid-filled body cavity,
completely enclosed by mesoderm. It lies between the digestive cavity and body wall (see Figure 1.3). Invertebrates
with a true coelom include mollusks and annelids.
Segmented Body
Segmentation evolved next. This is a division of the body into multiple segments. Both the earthworm and ant
pictured in Figure 1.4 have segmented bodies. This trait increases flexibility. It permits a wider range of motion.
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FIGURE 1.3
Cross Section of an Invertebrate with a
Coelom.
The coelom forms within the
mesoderm.
All annelids and arthropods are segmented. Arthropods also evolved jointed appendages. For example, they evolved
jointed legs for walking and “feelers” (antennae) for sensing.
FIGURE 1.4
Segmented Invertebrates. Earthworm (Annelid) and Black Ant (Arthropod). An earthworm consists of many small
segments. An ant has three larger segments. Notice the ants jointed legs and “feelers.”
Notochord
Some invertebrates evolved a notochord. This is the stiff support rod in a chordate. The first chordates were
probably similar to modern invertebrate chordates. The sea squirt in Figure 1.5 is an example. Later, some
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Chapter 1. Invertebrate Evolution
invertebrate chordates evolved into vertebrates.
FIGURE 1.5
Notochord. A sea squirt is an invertebrate
with a notochord.
Summary
• Many important traits evolved in invertebrates. They include: multicellularity, tissues and organs, radial
and bilateral symmetry, cephalization, mesoderm, complete digestive system, coelom, segmented body, and
notochord.
Explore More
Use this resource to answer the questions that follow.
• Invertebrate Diversity Part 1 - Porifera to Annelids at http://www.youtube.com/watch?v=CymI0LJquko .
1.
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Which animals do not have true tissues?
What is a Cnidarian? What is special about their structure?
Which animals have radial symmetry?
What are the three layers of cells? What forms from these layers?
Distinguish between acoelomates, pseudocoelomates, and coelomates.
Review
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Distinguish among asymmetry, radial symmetry, and bilateral symmetry.
Define cephalization. What is its relationship to bilateral symmetry?
What is mesoderm? Name an invertebrate with mesoderm.
Define coelom. What invertebrates have a true coelom?
What is segmentation? Why is it advantageous?
Compare and contrast incomplete and complete digestive systems. Why is a complete digestive system more
efficient?
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References
1. Laura Guerin, using sponge image by Pixabay:Josch13. Symmetry in invertebrates . CC BY-NC 3.0 (sponge
available in the public domain)
2. Christopher Auyeung. Three cell layers of a flatworm . CC BY-NC 3.0
3. Christopher Auyeung. Cross section of an invertebrate with coelom . CC BY-NC 3.0
4. Earthworm: Flickr:Dodo-Bird; Black ant: Pison Jaujip. Segmented invertebrates: earthworm and blackant .
CC BY 2.0
5. Silke Baron. Sea squirt . CC BY 2.0
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