recap

32.3 recap

Chordates are characterized by a dorsal hollow nerve chord, a post-anal tail, and a dorsal supporting rod called a notochord at some point during the life cycle. Specialized structures for support (a vertebral column), locomotion (such as fins), and feeding (jaws and teeth) evolved among aquatic vertebrates.

learning outcomes

You should be able to:

  • Recognize chordates and vertebrates, and distinguish them from other animals.

  • Use examples to assess whether hagfishes should be considered vertebrates.

  • Compare and contrast hagfishes and lampreys.

  • Describe the modifications and losses of appendages in different lineages of vertebrates and how these modifications are related to function.

Question 1

Describe the synapomorphies that characterize the chordates and the vertebrates, respectively.

Chordates are characterized by a dorsal hollow nerve cord, a post-anal tail, and a notochord. Vertebrates have an anterior skull that encloses a large brain, an internal skeleton supported by the vertebral column, internal organs that are suspended in the coelom, and a well-developed circulatory system driven by a ventral heart.

Question 2

How do the hagfishes differ from the lampreys in morphology? Why do some biologists contend that hagfishes are not vertebrates?

Hagfish have a weak circulatory system with three accessory hearts (rather than a single, large heart). They have only a partial skull and a simpler brain than other vertebrates, and they lack separate, jointed vertebrae in their skeletal system. Because hagfish lack fundamental traits shared by all other vertebrates, some biologists consider hagfish to be the sister group of vertebrates, rather than a member of the vertebrates. Another possibility is that these traits in hagfish have been secondarily lost or simplified.

Question 3

The body plan of most vertebrates is based on four appendages. What are the varied forms that these appendages take, and how are they used? In which lineages have two or more of these appendages been lost?

The four appendages common to most vertebrates are the two pectoral appendages and the two pelvic appendages. In most swimming vertebrates, these appendages function as fins. They are commonly used for propulsion (especially the pectoral fins) but are also used for steering, stabilization, and manipulation of the body position in water. Among tetrapods, the appendages are often modified into limbs used for walking, running, jumping, burrowing, climbing, grasping, and manipulating objects. There have been several reversals to finlike limbs in aquatic tetrapods (several times among amphibians, turtles, birds, and mammals, for example). The pectoral limbs of tetrapods were modified into wings for powered flight in at least three different lineages (birds, bats, and the extinct pterosaurs). The limbs have been modified for gliding as well (in fish, amphibians, lizards, and mammals). One or both pairs of appendages have been lost or greatly reduced in many groups of fish, amphibians, reptiles (including birds), and mammals. Some well-known examples of limb reduction or loss include the completely legless caecilians and snakes, the loss of external hindlimbs in whales and manatees, and the greatly reduced forelimbs of flightless birds.

In the lobe-limbed vertebrates, the gas-filled sacs that gave rise to swim bladders in ray-finned fishes became specialized for another purpose: breathing air. That adaptation set the stage for the vertebrates to move onto the land.