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Chordates - Advanced
Douglas Wilkin, Ph.D.
Jennifer Blanchette
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Printed: January 3, 2016
AUTHORS
Douglas Wilkin, Ph.D.
Jennifer Blanchette
www.ck12.org
C HAPTER
Chapter 1. Chordates - Advanced
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Chordates - Advanced
• Discuss the major characteristics of chordates, especially those that distinguish chordates from other phyla.
What’s a chordate?
Most chordates are animals with backbones. These range from small fish to giant whales and from playful dogs to
ferocious cats. They are not nearly as diverse as the invertebrates. But chordates do have many amazing species. The
white tiger (Panthera tigris) is a chordate. The tiger is also from the class Mammalia, order Carnivora, and family
Felidae, meaning it is a meat-eating cat.
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Characteristics of Chordates
The animal phylum Chordata includes the animals that we are by far the most familiar with: ourselves! We and
all other chordate species, including some quite primitive looking organisms, share several important features that
indicate our close evolutionary relationships. All of the phyla we have studied so far have consisted entirely of
invertebrate animals. The phylum Chordata includes both nonvertebrate and vertebrate groups. Some groups within
the chordates are considered to be very “young” compared to other chordates and to many of the phyla that we have
discussed in previous chapters. Birds, for example, have only been around for about 146 million years. Compare
that to some arthropods, such as horseshoe crabs, that have existed in basically the same form for over 500 million
years. In this lesson we will examine the specific features that define chordates. We will then consider the different
taxonomic groups within the phylum and how they evolved, keeping an emphasis on the nonvertebrate chordates.
Chordata is a large and diverse phylum that includes roughly 60,000 species. The examples of chordate species
shown in the Figure 1.1 illustrate their diversity and vast range in size. Chordates can be anywhere from about one
cm (the width of your index finger) to over 30 meters.
FIGURE 1.1
Different species of chordates, illustrating their diversity and vast range in
size. (a) A tunicate, approximately one inch in length. Notice the tiny fish
(also a chordate) swimming in front of the tunicate. (b) A white rhinoceros,
approximately 6600 pounds (3000 kg). (c) A kangaroo and joey. (d) A
great white shark. (e) A double-crested cormorant bird species. (f) A blue
whale.
Tunicates, for example, are small marine invertebrate members of the phylum that are generally one to a few
centimeters long. In contrast, some mammalian species can be quite large, either in length (for example, blue whales
can extend up to 30 meters in length) or in weight and girth (such as elephants, which can weigh up to roughly
20,000 lbs). Chordates can be found all over the planet, with species inhabiting marine, freshwater, terrestrial, and
aerial environments. They are found from the equator to the poles and everywhere in between. Members of the
phylum Chordata share the following characteristics (those traits specific to the phylum Chordata will be in bold):
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Three germ layers (triploblasts).
Bilateral symmetry.
Segmented body.
Coelom.
Complete digestive system.
Closed circulatory system.
Centralized nervous system.
Tail (post-anal).
Notochord.
Nerve cord.
Pharyngeal slits.
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Chapter 1. Chordates - Advanced
As you read this list, you may be thinking, “humans don’t have tails or slits in their pharynx!” But, in fact, we do.
Actually, to be more precise, we did. All chordates possess a tail and pharyngeal slits at some point in their lives,
and humans are no exception. Early on in human development, the embryo has both a tail and pharyngeal slits, both
of which are lost during the course of development. A human embryo with a prominent tail is shown in the Figure
1.2.
FIGURE 1.2
A five-week old (gestational age), ectopic
human embryo.
Notice the prominent
tail that extends beyond the anal region.
This embryo is ectopic, meaning it did
not successfully implant within the uterine
cavity. This complication is almost always
fatal to the embryo and is often dangerous
to the mother.
Chordates, like species in several other phyla such as annelids and arthropods, are bilaterally symmetrical triploblasts.
You will recall that bilaterally symmetrical animals can be divided into two equal, mirror-image halves when cut
down the middle of the anterior-posterior (front-back) axis. Triploblasts are organisms that develop from three basic
cell layers, called germ layers, in the embryo, which are called the endoderm, the mesoderm, and the ectoderm.
The endoderm forms the gut of the animal, the ectoderm forms the outer surface and the nervous system, and the
mesoderm forms the coelom and many of the internal organs including the muscles and circulatory system. Like
the annelids and arthropods, most chordates have at least partially segmented bodies. The segments are often highly
specialized into distinct body regions and do not resemble the repeating segments of annelids. In addition, chordates
are deuterostomes, meaning that they have a coelom (fluid-filled body cavity) that arises from internalized pouches
of tissue in the developing embryo and an anus that is formed from the embryonic blastopore. Chordate species
have a complete digestive system, and most species also have a closed circulatory system that relies on a ventral
(located on the bottom side of the organism’s interior) heart to pump blood throughout the body in enclosed blood
vessels. The chordate nervous system is usually highly centralized, with a distinct anterior brain that, in some groups
(including humans), is enclosed in a solid protective case called a cranium, or skull.
The four most prominent characteristics that distinguish chordates from species in all other phyla are the presence
of a post-anal tail, a notochord, a dorsal, hollow nerve cord, and pharyngeal slits (also called gill slits). The
organization of these features within the body plan of a chordate is illustrated in the Figure 1.3. The last three of
these features will be discussed in more detail in the following sections.
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FIGURE 1.3
The Body Plan of a Typical Chordate. The
body plan of a chordate includes a postanal tail, a notochord, a dorsal, hollow
nerve cord, and pharyngeal slits.
Notochord
The most important thing to remember about a notochord is that it is not a nerve cord or a backbone (column of
vertebrae). A backbone is the distinguishing feature of the subphylum Vertebrata within the phylum Chordata (this
subphylum will be discussed in the next section). It is easy to confuse the notochord with either of these because of
its name and its function. The notochord is a long, rod-like support structure that usually extends the length of the
animal. As shown in the Figure 1.3, the notochord is situated between the gut (it is dorsal to the gut) and the dorsal
nerve cord (it is ventral to the nerve cord). Chordates are defined as having a notochord at some point during their
lives, but they do not all retain the notochord as adults. In some groups it is completely lost, in others (generally
nonvertebrate chordate species) it is retained and functions to support the body of the animal during locomotion.
In adult vertebrates, the notochord is essentially replaced by the backbone, however, the notochord should not be
considered a primitive backbone. It is a separate structure entirely. All chordates have a notochord during embryonic
development when it plays an important role in specifying the organization of the developing embryo. For example,
the presence of the notochord communicates to nearby cells that they should differentiate in certain ways based on
their position relative to the notochord.
Nerve Cord
A dorsal (top side), hollow nerve cord derived from ectodermal tissue is another characteristic that distinguishes
chordates from other animals. Most other bilateral animal phyla with a nervous system have a solid, ventral (bottom
side) nerve cord. Unlike the notochord, essentially all vertebrate chordates retain the nerve cord as adults. The nerve
cord ultimately develops into the brain and spinal cord in most adult vertebrates. It is the core of the centralized
nervous system.
Pharyngeal Slits
Like the notochord, pharyngeal slits are a feature shared by all developing chordate embryos but not by all chordate
adults. A pharynx is the region of the digestive tract that functions to connect the mouth to the esophagus. In many
animals, including humans, it is also a junction between the respiratory system and the digestive system, connecting
the mouth (digestive opening) and nasal passages to the bronchial tubes of the lungs. The pharyngeal slits are formed
from invaginations, or pouches, in the lining of the pharynx. The developing pharyngeal pouches eventually form
openings through the pharyngeal wall called slits. This is shown in the Figure 1.4. The slits function in feeding
and also in respiration in some species. Water flows through the slits as a result of the action of beating hair-like
projections, called cilia, that line the slits, and food particles are trapped for ingestion. In most aquatic species, the
pharyngeal slits ultimately develop into the gills of the adult organism, and they are used to absorb oxygen from the
incoming water. In non-aquatic species, the pouches/slits develop into other structures connected to the pharyngeal
region including components of the middle ear and the tonsils in humans.
Now that we have examined the overall characteristics of chordates, let’s consider how species within the phylum
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Chapter 1. Chordates - Advanced
FIGURE 1.4
A diagram of a cross-section through a chordate pharynx. The developing
pharyngeal pouches on the right side are labeled 1 through 4. As the
pouches extend to the outer edge of the pharynx, they will eventually extend through the pharyngeal wall to form slits, as shown in the uppermost
pouch on the left.
are classified with regard to their evolutionary history.
Vocabulary
• blastopore: The opening of an embryo’s central cavity in the early stage of development.
• cilia (singular, cilium): Short hairlike projection, similar to flagella, that allow some cells to move.
• coelom: A fluid-filled cavity formed within the mesoderm; it forms between the digestive cavity and the body
wall.
• cranium: The skull of a vertebrate.
• deuterostome: An animal in which the first opening formed during development (the blastopore) becomes
the anus.
• nerve cord: A single hollow tract of nervous tissue; it forms the central nervous system of chordates and
develops into the spinal cord and brain in vertebrates.
• notochord: A rod-shaped, semi-rigid support structure that forms between the dorsal nerve cord and the gut
of an animal.
• pharyngeal slits: Openings between the pharynx, or throat, and the outside.
• triploblast: An animal with three germ layers: an endoderm, a mesoderm, and an ectoderm.
Summary
• Chordates are bilaterally symmetrical triploblasts with true coeloms and segmented bodies.
• Like echinoderms, chordates are deuterostomes.
• The four most prominent characteristics that distinguish chordates from species in all other phyla are the
presence of a post-anal tail, a notochord, a dorsal, hollow nerve cord, and pharyngeal slits (also called gill
slits).
• The notochord is a long, rod-like support structure that usually extends the length of the animal.
• While a notochord and pharyngeal slits might not be present in all adult chordates, all vertebrate chordates
retain the nerve cord as adults.
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Practice
Use this resource to answer the questions that follow.
• Chordate at http://www.britannica.com/EBchecked/topic/114462/chordate .
1. How do the pharyngeal slits function in tunicates and cephalochordates? How does this differ from vertebrates?
2. How do chordates reproduce?
3. What function do notocords serve in locomotion?
Review
1.
2.
3.
4.
5.
What are the four features that distinguish chordates from other phyla?
As a human develops, which chordate characteristics do we lose as we turn into adults?
What important role does the notochord play in embryonic development?
How does the nerve cord of chordates differ from the nervous systems of other animals?
What function do the pharyngeal slits serve?
References
1. Tunicate: Chika Watanabe; Rhinoceros: Brian Snelson; Kangaroo: Subhash Chandra; Shark: Terry Goss; Cormorant: Linda Tanner; Whale: Courtesy of US National Oceanic and Atmospheric Administration. Tunicate:
http://www.flickr.com/photos/chikawatanabe/61157859/; Rhinoceros: http://www.flickr.com/photos/exfordy
/2961323775/; Kangaroo: http://www.flickr.com/photos/ysc/2179798100/; Shark: http://commons.wikimedi
a.org/wiki/File:White_shark.jpg; Cormorant: http://www.flickr.com/photos/goingslo/2306021399/in/photostr
eam/; Whale: http://www.nmfs.noaa.gov/pr/species/mammals/cetaceans/bluewhale.htm . Tunicate, Rhinoceros,
Kangaroo, Shark, Cormorant: CC BY 2.0; Whale: Public Domain
2. Ed Uthman, MD. http://commons.wikimedia.org/wiki/File:Tubal_Pregnancy_with_embryo.jpg . Public Domain
3. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. Body plan of a typical chordate . CC BY-NC
3.0
4. Uwe Gille. http://en.wikipedia.org/wiki/Image:Kiemenbogen.jpg . CC-BY-SA
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