Bilaterian animals are classified into two major clades, protostomes and deuterostomes. The non-
learning outcomes
You should be able to:
Apply knowledge of phylogenetic relationships among major groups of animals to describe the evolution and loss of major structural features.
Apply information about the life histories of major animal groups to design methods for studying them.
What is the basis for the claim that animal nervous systems evolved independently in ctenophores, cnidarians, and bilaterians?
First, phylogenetic analysis showed that ctenophores were the sister group of other animals. Ctenophores have nervous systems, but some other animals (such as sponges and placozoans) do not. Therefore either nervous systems evolved once in the ancestor of all animals and were subsequently lost in sponges and placozoans, or nervous systems evolved separately in ctenophores and in other animals. Second, analysis of whole genomes showed that the genes involved in nervous systems have been independently duplicated and specialized in ctenophores, cnidarians, and bilaterians, thereby suggesting that the nerve nets of ctenophores and cnidarians, and the centralized nervous systems of bilaterians, each evolved independently.
Why are sponges and placozoans considered to be animals even though they lack the complex body structures found in most other animal groups?
The group of organisms called “animals” represents a specific monophyletic group of multicellular organisms on the tree of life. Sponges retain many of the ancestral features of animal relatives (such as choanoflagellates), and split from the other animals before the evolution of complex organ systems. Placozoans, by contrast, may have evolved from ancestors with distinct organ systems, but they appear to have lost these systems and become secondarily simplified.
The discoveries that the pelagic stages of placozoans are abundant in warm seas and that the mature stages settle on smooth surfaces suggest how these organisms might be collected and surveyed. What sampling procedures might you use to discover whether placozoans occur at a particular location along a coast?
Placement of glass microscope slides (or other smooth substrates for placozoan attachment) in warm tropical waters often results in colonization by placozoans. The glass slides can be suspended in water in survey areas, then later retrieved and examined for the presence of placozoans.
Activity 30.3 Diversity Simulation
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