BIOL 1500 Lecture Notes - Lecture 17: Symmetry In Biology, Gastrulation, Body Cavity
6 Oct 2016
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Animals are diverse, yet they all have features in common: multicellular bodies, eukaryotic cells that lack cell walls, heterotrophic by ingestion, blastula (hollow ball) stage of development, cells bind to extracellular matrix. The first animals arose from aquatic protists about 570 million years ago. They came before plants, plants arose 475 million years ago. Animals are classified into 9 major phyla. The first branched point in animal taxonomy distinguishes a clade with true tissues from a clade with no true tissues. A tissue is a collection of specialized cells that function in a coordinated fashion. In sponges, cells do not interact to provide specific functions, so sponges do not have true tissues. The second branching point distinguishes radically symmetrical from bilaterally symmetrical animals. An organism has radial symmetry if any plane passing through the body from the mouth to the opposite end creates mirror images.
