BIO 101 Lecture Notes - Lecture 24: Symmetry In Biology, Germ Layer, Jellyfish
25 Jan 2017
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Document Summary
These characteristics, together, distinguish animals from members of other taxonomic groups. Animals obtain energy by consuming other organisms. Animals are motile at some point in the life cycle. Most animals are able to respond rapidly to external stimuli. One of the earliest innovations in animal evolution was the appearance of tissues. Tissues are groups of similar cells integrated into a functional unit (muscle tissue, nerve tissue, etc. ) Sponges lack of tissues separates them from all other animals. The appearance of tissues coincided with the appearance of body symmetry. The difference between radially and bilaterally symmetrical animals reflects another major branching point in the animal evolutionary tree. This split separated the ancestors of radially symmetrical cnidarians from the ancestors of the remaining animal phyla, all of which are bilaterally symmetrical. Radially symmetrical animals have two embryonic tissue layers (germ layers) Endoderm is an inner germ layer, which gives rise to tissues that line the gut cavity.
