recap

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32.1 recap

The three major clades of deuterostomes are the echinoderms, the hemichordates, and the chordates. The common ancestry of these groups is supported by early developmental similarities and by phylogenetic analyses of DNA sequences.

learning outcomes

You should be able to:

  • Summarize the developmental and molecular characteristics that distinguish deuterostomes.

  • Identify the major deuterostome clades and give examples of each.

Question 1

What three developmental traits do all deuterostomes have in common? Why are some of these traits no longer considered to be strong evidence for the monophyly of deuterostomes?

Deuterostomes have radial cleavage, development of the blastopore into an anus (with formation of the mouth at the opposite end of the embryo from the blastopore), and development of the coelom from mesodermal pockets that bud off from the cavity of the gastrula. However, radial cleavage is thought to be the ancestral condition for all bilaterians and not unique to deuterostomes, as it is also found among some protostomes. The development of the blastopore into an anus is unique to deuterostomes, but it may also represent the ancestral condition for bilaterians.

Question 2

What evidence provides strong support for the monophyly of deuterostomes?

DNA sequences of many different genes provide the strongest support for the monophyly of deuterostomes.

Question 3

What are the three major groups of deuterostomes? Place each of the following in the appropriate group: humans, acorn worms, rattlesnakes, tunas, and sea stars.

The three major groups of deuterostomes are echinoderms, hemichordates, and chordates. Sea stars are echinoderms, acorn worms are hemichordates, and humans, rattlesnakes, and tunas are chordates.

We will begin our survey of the deuterostomes with the echinoderms and hemichordates, the most distant of our relatives among the deuterostomes.