Topic 11: Introduction to Bilateria, Protostomes, and Lophotrochozoans (Ch. 33) Bilateria

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Topic 11: Introduction to Bilateria, Protostomes, and Lophotrochozoans (Ch. 33)

  1. Bilateria (clade)

    1. have bilateral symmetry at some point in their life cycle

    2. all are triploblastic

    3. all the rest of the animal phyla that we will cover are in this group

    4. traditional taxonomy note:

      1. were once generally divided into acoelomates, pseudocoelomates, and coelomates

      2. acoelomates and pseudocoelomates sometimes called lower bilateria

      3. however, based on molecular evidence (sequencing of ribosomal RNA genes) it appears that grouping into acoelomate, pseudocoelomate, and coelomate bilateria produces polyphyletic groups, so we do not use it anymore

      4. even so, the traditional taxonomy is so firmly entrenched that it is worth knowing about

    5. Most members of Bilateria fall into one of two large clades, Deuterostomia (deuterostomes) and Protostomia (protostomes)

    6. we will cover one exception to the two main clades: arrow worms

  1. Phylum Chaetognatha (arrow worms) [clade]

    1. ~70 living species; abundant predators in marine plankton

    2. coelomate

    3. translucent

    4. centimeter size range

    5. arrow shape with head, trunk, and tail segments and septa between these

    6. large eyes and powerful jaws in some

    7. date back to 500 MYA in fossil record

  1. Protostomia (clade)

    1. most have spiral, determinate cleavage; coelom form from splits within mesoderm; and blastopore become mouth

    2. made up of two “sister taxa”: Ecdysozoa (molting) and Lophotrochozoa (non-molting)

  1. Lophotrochozoa (clade)

    1. grow by extending the size of their skeletons; do not molt

    2. group is unified mostly by genetic similarities and lack of molting

    3. name comes from two things found in some animals in the group

      1. lophophore

      1. trochophore

    1. clade includes the following phyla that we will cover:

      1. Ectoprocta (or Bryozoa; clade)

      2. Platyhelminthes (clade?)

      3. Rotifera (clade)

      4. Annelida (clade)

      5. Nemertea (clade)

      6. Brachiopoda (clade?)

      7. Phoronida (clade)

      8. Mollusca (clade)


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