Organization of the Animal Bod1

Organization of the Animal Bod1 - Evolution and...

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Organization of the Animal Body Animals are characteristically multicellular heterotrophs whose cells lack cell walls . At some point during their lives, animals are capable of movement. In the most commonly encountered animals, this stage is the adult, although some animals (corals) have sessile (nonmobile) adult phases and mobile juvenile forms. Animal and plant evolutionary history both show the development of multicellularity and the move from water to land (as well as secondary adaptation back to water). Animals developed external or internal skeletons to provide support, skin to prevent or lessen water loss, muscles that allowed them to move in search of food, brains and nervous systems for integration of stimuli, and internal digestive systems . Most animals have a life cycle with a preadult stage, a predominance of the diploid stage, and a series of embryonic developmental stages.
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Unformatted text preview: Evolution and Classification of Animals Animals probably evolved from marine protists , although no group of protists has been identified from an at-best sketchy fossil record for early animals. Cells in primitive animals (sponges in particular) show similarities to collared choanoflagellates as well as pseudopod-producing amoeboid cells. Multicellular animal fossils and burrows (presumably made by multicellular animals) first appear nearly 700 million years ago, during the late precambrian time (the part of the Proterozoic era termed the Vendian). All known Vendian animal fossils had soft body parts: no shells or hard (and hence preservable as fossils) parts. Learn more about these early animal fossils at Learning About the Vendian Animals . Animals in numerous phyla appear at (or in many cases before) the beginning of the Cambrian Period, as shown in Figure 1....
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