27.3 Animal Phylogeny

27.3 Animal Phylogeny

  • There are two layers of muscle under the skin, but there is no muscle around the gut.
    • Flatworms rely on passive diffusion for their nutrition.
  • There is a body wall between the gut and the body wall in pseudocoelomates.
    • The mesoderm in these animals doesn't develop cavities.
    • The rotifers and nematodes are pseudocoelomates.
    • Eucoelomates are animals that have a true coelom, and include all mammals, birds, arthropods, and echinoderms.
    • During embryogenesis, the coelom develops.
    • The molluscs, annelids, and arthropods are schizocoels, in which the mesoderm splits to form the body cavity.
    • The body cavity is formed by the buds that separate from the gut.
    • mammals have a body that is divided into two parts, the abdominal and the thoracic.
    • The pseudocoelomates may have lost their ability to form a complete coelom because of genetic changes.
    • The formation of the coelom has had a large evolutionary impact on the various species of the animal kingdom.
  • Biologists try to understand the evolutionary history of members of the animal kingdom and all of life.
    • The branching sequence of evolution is the focus of the study.
    • The animal kingdom is divided into 35 to 40 phyla.
    • Pequeas are hypotheses about which species have evolved from which ancestors.
  • Until recently, only the fossil record was used to determine relationships among animals.
    • Scientific understanding of the distinctions between the different parts of the body gave much of the knowledge.
    • This information can be misleading if used alone.
    • Animals may have similar characteristics, but their evolution may be different.
    • Modern phylogenetics is informed by genetic and molecular analyses, in addition to traditional fossil data.
    • With a growing understanding of genetics, the animal evolutionary tree has changed substantially and continues to change as new DNA and RNA analyses are performed on additional animal species.
  • There are similarities between the feeding cells of sponges and choanoflagellate protists.
  • The choanocyte flagella draws water through the sponge so that it can be removed.
  • Bilateria and Radiata are the clades of Eumetazoa, which are radially symmetrical animals and bilaterally symmetrical animals.
    • The cnidarians and ctenophores are animals with true radial, biradial, or rotational symmetry.
    • The Bilateria clade has all the other Eumetazoa.
    • There are two distinct clades of Protostomes, including ecdysozoans and lophotrochozoans, in Figure 27.13a,b.
    • Some lophotrochozoan phyla are characterized by a feeding structure called a lophophore, which is the shorter term, "lopho-trochozoa".
  • The lophophore is a feeding structure.
  • Click to learn more about the organisms and their evolutionary relationships.
  • Evolutionary biologists are debating and refining the groupings.
    • New evidence changes the relationships described in a tree diagram.
  • The following video shows how biologists use genetic data to determine relationships among organisms.
  • The modern phylogenetic animal tree has been greatly modified by the use of genes.
    • Some of the data come from a variety of sources.
    • The modern tree has many evolutionary relationships that have only recently been determined.
  • A group of animals called lophophorates, which included brachiopods and bryozoans, were long thought to be primitive.
    • These animals are related to annelids and mollusks in many ways.
    • The discovery allowed for the distinction of the clade.
    • The placement of the Platyhelminthes is problematic because of some differences within the lophotrochozoan group.
    • Some scientists think that the phyla Platyhelminthes should be part of their own clade.