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13.1: Introduction to Vertebrates

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Lesson Objectives

  • 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 an invertebrate?
  • What is a vertebrate?


  • cranium
  • endostyle
  • notochord


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 below). The urochordates consist of 3,000 species of tunicates, sessile marine animals with sack-like 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 below). Cephalochordates consist of 30 species of lancelets (burrowing marine animals).
  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.

Tunicate colonies of Botrylloides violaceus (subphylum Urochordata) have tentacles at openings of tubes that take in food and water and release waste and water.

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 vertebrates 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?

Typical vertebrate traits include:

  • 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 below).

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 below).

Type of Tetrapod Number of Species
Amphibians 6,000
Reptiles 8,225
Birds 10,000
Mammal-like Reptiles 4,500
Mammals 5,800

Lesson Summary

  • 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


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

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?

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Date Created:

Feb 29, 2012

Last Modified:

Sep 02, 2014
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