31.1 Animals are multicellular heterotrophs without cell walls.
Some General Features of Animals
• Animals are multicellular heterotrophs that are diverse in form and habitat, are mobile, reproduce sexually, and have characteristic embryonic development. (p. 618)
31.2 Animals are a very diverse kingdom.
The Traditional Classification of Animals
• Taxonomists have traditionally created phylogenies by comparing anatomical features and embryological development. (p. 620)
• Kingdom Animalia is traditionally divided into the Parazoa, which lack a definite symmetry and organized tissues, and the Eumetazoa, which have a definite symmetry and organized tissues. (p. 620)
• The eumetazoan branch is divided into Radiata and Bilateria. (p. 620)
• Bilateral animals further split into groups with and without a body cavity. (p. 620)
31.3 The animal body plan has undergone many changes.
Five Key Transitions in Body Plan
• The evolution of tissues involved cell specialization. (p. 624)
• The evolution of bilateral symmetry allowed organization of body parts, including cephalization, and increased motility. Bilaterally symmetrical animals produce three germ layers -- ectoderm, endoderm, and mesoderm. (p. 624)
• The evolution of a body cavity allowed the subsequent development of efficient organ systems, such as an open or a closed circulatory system. Three basic types of body plans are (1) the acoelomates with no body cavity, (2) the pseudocoelomates with a cavity between the mesoderm and endoderm, and (3) the coelomates with a fluid-filled body cavity entirely within the mesoderm. (p. 625)
• The evolution of deuterostome development involved changes in cleavage patterns, determination, the fate of the blastopore, and the formation of the coelom. (pp. 626-627)
• The evolution of segmentation allowed the duplication of body organs and more effective locomotion. (p. 627)
31.4 The way we classify animals is being reevaluated.
A New Look at the Metazoan Family Tree
• Traditional characters used to construct phylogenies are not as conservative as once supposed. (p. 628)
• Molecular systematics uses unique sequences within certain genes to identify shared characteristics that define the monophyletic taxa making up clades. (p. 628)
"Evo-Devo" and the Roots of the Animal Family Tree
• The multinucleate hypothesis, the colonial flagellate hypothesis, and the polyphyletic origin hypothesis all try to account for the origin of metazoans from single-celled protists. (p. 630)
• A large diversity of animal body plans occurred around the Cambrian period, with no new innovations since. (p. 630)
• The emergence of body plans has been hypothesized to be caused by the emergence of predatory lifestyles, geological factors, and changes in the location or time of expression of Hox genes within developing animal embryos. (p. 630)