Introduction to Vertebrates
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Printed: August 11, 2014
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C HAPTER
Chapter 1. Introduction to Vertebrates
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Introduction to Vertebrates
Lesson Objectives
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Describe the general features of chordates.
List the three groups of chordates and their characteristics.
List the general features of vertebrates.
Describe the classification of vertebrates.
Check Your Understanding
• What is a invertebrate?
• What is a vertebrate?
Vocabulary
• cranium
• endostyle
• notochord
Chordates
Did you know that fish, amphibians, reptiles, birds and mammals are all related? They are all chordates. Chordates
are a group of animals that includes vertebrates, as well as several closely related invertebrates. All chordates
(phylum Chordata) have a notochord, a hollow nerve cord along the back.
They also have:
1. Pharyngeal slits, which help to filter out food particles.
2. An endostyle, which has small hairs and is used to gather food particles and move them along the digestive
tract.
3. A post-anal tail, which is present during the lifetimes of some chordates and during the development of others.
The chordate phylum is broken down into three subphyla:
1. Urochordata (represented by tunicates): Urochordates have a notochord and nerve cord only during the larval
stage ( Figure 1.1). The urochordates consist of 3,000 species of tunicates, sessile marine animals with sacklike bodies and tubes for water movement.
2. Cephalochordata (represented by lancelets): Cephalochordates have a notochord and nerve cord but no vertebrae, or bones in the backbone ( Figure 1.2). Cephalochordates consist of 30 species of lancelets (burrowing
marine animals).
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3. Vertebrata (the vertebrates): Humans fall in this category. In all vertebrates except for hagfish, the notochord
is smaller and surrounded by vertebrae. Vertebrates all have backbones or spinal columns. About 58,000
species have been described, including many familiar groups of large land animals.
The origin of chordates is currently unknown. The first clearly identifiable chordates appear in the Cambrian Period
(about 542 - 488 million years ago) as lancelet-like specimens.
FIGURE 1.1
Tunicate colonies of Botrylloides violaceus (subphylum Urochordata) have
tentacles at openings of tubes that take
in food and water and release waste and
water.
FIGURE 1.2
Pikaia gracilens (subphylum Cephalochordates), perhaps the oldest known ancestor of modern vertebrates, resembled
a living chordate, known as a lancelet,
and perhaps swam much like an eel.
What other modern-day organisms does
a pikaia look like?
What are Vertebrates?
Vertebrates, in the subphylum Vertebrata, are chordates with a backbone. Vertebrates have a braincase, or cranium,
and an internal skeleton (except for lampreys). You can tell the difference between verebrates and other chordates
by looking at their head. Vertebrates have cephalization. Cephalization means an organism’s nervous tissue is found
toward one end of the organism. In other words, this is like having eyes in your head. Why do you think this type of
body design is an advantage?
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Chapter 1. Introduction to Vertebrates
Typical vertebrate traits include:
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A backbone or spinal column.
Cranium.
Internal skeleton.
Defined head with pronounced cephalization.
Sensory organs, especially eyes.
Living vertebrates range in size from a carp species, as little as 0.3 inches, to the blue whale, as large as 110 feet (
Figure 1.3).
FIGURE 1.3
Vertebrates vary in size.
Classification of Vertebrates
Vertebrates can be divided into two major groups: those with or without jaws. There are more than 100 species of
jawless vertebrate. There are more than 50,000 species of vertebrate with jaws.
The jawed vertebrates include species of fish with cartilage, the strong, flexible tissue found in human ears, bony
fish, and four-limbed animals. These animals are known as tetrapods. Some tetrapods include amphibians, reptiles,
birds, and mammals ( Table 1.1).
TABLE 1.1: Species of the Main Groups of Tetrapods
Type of Tetrapod
Amphibians
Reptiles
Birds
Mammal-like Reptiles
Mammals
Number of Species
6,000
8,225
10,000
4,500
5,800
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Lesson Summary
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Chordates are characterized by a notochord.
There are three main groups of chordates, including tunicates, lancelets and vertebrates.
Vertebrates are distinguished by having a backbone or spinal column.
Vertebrates are classified into two major groups: those without jaws and those with jaws.
Review Questions
Recall
1. What is the main feature that characterizes the chordates?
2. What are the main features of vertebrates?
Apply Concepts
3. Which two structures that all chordates possess sometime during their life cycle (during development or otherwise)
are used for food gathering, and how are these structures used?
4. Why do you think cephalization is not necessary in urochordates and cephalochordates? Explain how this is
illustrated in tunicates.
Critical Thinking
5. The first clearly-identifiable chordates are lancelet-like (small, burrowing marine animals with a lancet shape)
specimens. Propose one way that these first chordates could have evolved into a swimming-like animal.
Further Reading / Supplemental Links
• http://www.ucmp.berkeley.edu/chordata/chordata.html
• http://www.ucmp.berkeley.edu/vertebrates/vertintro.html
Points to Consider
• How do you think a notochord could help fish adapt to swimming?
• How do you think cephalization could be an advantage in movement and feeding in fish?
References
1. Courtesy of Dann Blackwood, USGS. http://commons.wikimedia.org/wiki/File:Botrylloides_violaceus.jpg .
Public Domain
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Chapter 1. Introduction to Vertebrates
2. Nobu Tamura. http://commons.wikimedia.org/wiki/File:Pikaia_BW.jpg . CC BY 3.0
3. Laura Guerin. CK-12 Foundation . CC BY-NC 3.0
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