Members of the subphylum Vertebrata, within the phylum Chordata (kawr-DAY-tuh) have, at some stage of life, a notochord (a rod-shaped supporting axis below the nerve cord), a dorsal hollow nerve cord, pharyngeal pouches (paired structures in the throat region), and a post-anal tail (a tail that extends beyond the anus). Vertebrates have three characteristics that distinguish them from other chor-dates. First, vertebrates have vertebrae (singular, vertebra), bones or cartilage that surround the dorsal nerve cord and form the spine. Second, vertebrates have a cranium, or skull, that protects the brain. Third, all vertebrates have an endoskeleton (an internal skeleton) composed of bone or cartilage.

Vertebrate Classes

Today, there are about 45,000 species of vertebrates. They occupy all but the most extreme terrestrial habitats. The nine classes of vertebrates are summarized below.

• Hagfishes (class Myxini)—These fishes have elongated, eel-like bodies. They lack jaws, paired fins, and bone. The notochord remains throughout life. Hagfishes do not have vertebrae. Many scientists do not consider them vertebrates. They are included with vertebrates because they do have a cranium and an endoskeleton.

• Lampreys (class Cephalaspidomorphi)—Lampreys lack jaws, paired fins and bone, and retain a notochord throughout life. However, unlike hagfishes, lampreys have a primitive vertebral column composed of cartilage that surrounds the notochord.

• Sharks, Rays, Skates, and Ratfishes (class Chondrichthyes)— These predatory fishes have jaws and paired fins. Their skeleton is made of cartilage, not bone, and many have skin covered by a unique kind of scale.

• Ray-finned Fishes (class Actinopterygii)—Most familiar fishes are ray-finned fishes. All have jaws and paired fins, most have a skeleton composed of bone. These fish have fins supported by rays of bone that fan out from a central bony axis.


• Identify the distinguishing characteristics of vertebrates.

• List an example for each of the nine classes of vertebrates.

• Describe the characteristics of the early vertebrates.

• Explain the importance of jaws and paired fins for fishes.

vocabulary vertebrae cranium gill arches

Word Roots and Origins vertebra from the Latin vertebra, meaning "a joint"

□ inieriritictoriictt Topic: Fishes Keyword: HM60579

□ inieriritictoriictt Topic: Fishes Keyword: HM60579

Maintained by the f IAJ f C National Science H\I\j. Teachers Aisoclatlon

Ancestral chordate figure 39-1

This phylogenetic diagram shows hypotheses of the evolutionary relationships among the many diverse groups of chordates. For updates on phylogenetic information, visit and enter the keyword HM6 Phylo.

Analyzing a Phylogenetic Tree

Materials paper, pencil


1. Draw the phylogenetic tree shown on this page on your paper.

2. Using the information on pp. 779 and 780, determine the key characteristics that distinguish each vertebrate group. Indicate these evolutionary changes on the branches of the tree to make a diagram of the relationship that exists among vertebrates. Begin at the bottom of the tree with the key characteristics that distinguish vertebrates from other chordates. Analysis Which characteristics are shared by all vertebrates? Which key characteristic separates the classes Chondrichthyes and Actinopterygii? Which adaptations led to the divergence of mammals?

Class Myxini







(sharks, skates, rays, and ratfishes)



(ray-finned fishes)

Class Sarcopterygii

(lobe-finned fishes)

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