rav65819_ch33_635-652 - 33 chapter Noncoelomate...

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;;;;;;;; 33 chapter Noncoelomate Invertebrates introduction WE START OUR EXPLORATION of the great diversity of animals with the simplest members of the animal kingdom—sponges, jellyfish, and simple worms. These animals lack a body cavity
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(coelom), and they are thus called noncoelomates. The major organization of the animal body first evolved in these animals, a basic body plan on which all the rest of animal evolution has depended. Although traditionally systematists considered noncoelomate animals to be closely related, we now realize that this is not the case, as we discussed in the preceding chapter. In chapter 34, we consider the invertebrate animals that have a coelom, and in chapter 35 the vertebrates. You will see that all animals, despite their great diversity, have much in common. concept outline 33.4 The Bilaterian Acoelomates Cnidarians are grouped into four classes The comb jellies, phylum Ctenophora, use cilia for movement 33.1 A Revolution in Invertebrate Phylogeny Both traditional and novel phylogenies agree on major groupings The traditional phylogeny focuses on the state of the coelom The novel protostome phylogeny distinguishes spiralians from ecdysozoans 32.2 Parazoa: Animals That Lack Specialized Tissues The sponges, phylum Porifera, have a loose body organization The sponge body is composed of several cell types Choanocytes circulate water through the sponge Sponges can reproduce both asexually and sexually 33.3 Eumetazoa: Animals with True Tissues The cnidarians, phylum Cnidaria, exhibit internal extracellular digestion The flatworms, phylum Platyhelminthes, have an incomplete gut or none at all Flatworms have been traditionally grouped into four major classes Acoel flatworms appear to be distinct from Platyhelminthes: A case study The ribbon worms, phylum Nemertea, are not closely related to other acoelomates Phylum Cycliophora is a relatively new phylum 33.5 The Pseudocoelomates The roundworms, phylum Nematoda, are ecdysozoans comprising many species The rotifers, phylum Rotifera, move by rapidly beating cilia
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635 rav65819_ch33_635-652.indd 635 rav65819_ch33_635-652.indd 635 12/7/06 2:04:36 PM 12/7/06 2:04:36 PM 33.1 A Revolution in Invertebrate Phylogeny There is little disagreement among biologists about the taxonomic classification of animals. For example, an annelid worm would be classified in the phylum Annelida by any competent taxonomist. Great disagreement exists, however, about how the animal phyla are related to one another. Depending on what aspects of the phyla are compared, different biologists draw quite different family trees. Both traditional and novel phylogenies agree on major groupings For many years, biologists have based their reconstructions of the animal tree of life on key aspects of body architecture, lumping together those phyla that share fundamental aspects of body plan. For the better part of a century, biologists agreed on the principal aspects of this tree, basing the phylogeny largely on anatomical and embryological comparisons.
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