key concept 32.3 Chordates Have a Dorsal Nerve Cord and a Notochord

As you have seen, it is not obvious from examining adult animals that echinoderms and chordates share a common ancestor. The evolutionary relationships among some chordate groups are not immediately apparent either. The features that reveal all of these evolutionary relationships are seen primarily in the larvae—in other words, it is during the early developmental stages that these evolutionary relationships are evident.

focus your learning

  • Chordates are characterized by a dorsal hollow nerve cord, a post-anal tail, and a dorsal supporting rod called a notochord.

  • Vertebrates are distinguished by an additional dorsal supporting structure—the vertebral column—and other features.

  • Hagfishes have weak circulatory systems and lack, or have reductions in, many otherwise shared vertebrate features.

  • Lampreys and hagfishes may look superficially similar, but they differ greatly in their biology.

  • Ray-finned fishes evolved swim bladders, which are organs of buoyancy.

There are three principal chordate clades: the lancelets (also called cephalochordates), the tunicates (also called urochordates), and the vertebrates (see Figure 32.1). Adult chordates vary greatly in form, but all chordates display the following derived structures at some stage in their development (Figure 32.7):

image
Figure 32.7 Key Features May Be Most Apparent in Early Development (A) The tunicate larva (but not the adult) has all three key features of chordates: a dorsal hollow nerve cord, a post-anal tail, and a notochord. (B) All three chordate synapomorphies are retained in the adult lancelet.

The notochord is the most distinctive derived chordate trait. It is composed of a core of large cells with turgid fluid-filled vacuoles, which make it rigid but flexible. In the tunicates the notochord is lost during metamorphosis to the adult stage. In most vertebrate species it is replaced during development by skeletal structures that provide support for the body.

690

The pharyngeal slits found in the common ancestor of deuterostomes are present at some developmental stage in all chordates but are often lost or greatly modified in adults. In chordates, the pharyngeal slits are separated and supported by structural elements called pharyngeal arches. In tunicates and lancelets, the pharynx functions as a straining device to filter small food particles. In fishes and larval amphibians, some of the pharyngeal arches develop into gill arches, which support the respiratory gills and are often used as feeding structures as well. Developmentally, some pharyngeal arches also develop into elements of the vertebrate jaw, as well as parts of the tongue, larynx, trachea, and middle ear of tetrapods (four-legged vertebrates). Some of the pharyngeal slits are modified in tetrapods to form the eustachian tube and middle ear chamber.