26.1 An Overview of Animals

	The animal gets its shape and structure from its skeletal system.
	Sessile organisms permanently attached to one spot.
	Most sessile animals have a motile stage in their life cycle.
	Early animals left no fossils, yet they may have originated 700-800 million years ago.

26.2 Body Plans

	Most body plans of animals are compared on the basis of symmetry and segmentation.
	Dorsal - top surface.
	Ventral - bottom surface.
	Bilateral symmetry - body parts arranged in pairs on either side of central axis.
	Anterior - front end.
	Posterior - rear end.
	Sense organs tend to be concentrated at the anterior end of bilaterally symmetrical animals.
	Segmentation is a common characteristic in the body plan.
	Segmentation isn't obvious from the outside.
	The structure of the spine reveals the segmentation of the body.
	Each vertebra bone represents a segment.
	Segmentation enables specialization of body parts and more complex bodies.

26.3 Tissue Development

	The fertilized egg, zygote, divides by the process called cleavage.
	The embryo becomes a hollow ball of cells called blastula.
	Blastula develops where cells move inward from the surface, forming a two-layered cup called a gastrula.
	Mesoderm forms a fully lined body cavity called coelom.
	All animals don't develop a coelom.
	Flatworms have no body cavity other than a digestive tract.
	Coelom provides place for development of more complex organ systems.
	Protostome - blastopore that develops into a mouth.
	All vertebrates, including sea stars, are deuterostomes.
	Echinoderms are closest relatives of chordates by studying embryos because animals in both phyla are deuterostomes.

26.4 Organized Systems

	Animals need a support system firm enough to support but flexible enough to move.
	Hard encasement on the surface of an animal is an exoskeleton.
	Digestive cavity of a hydra has a single opening.
	It allows food to enter and wastes to leave through the same opening.
	A cluster of nerve cell bodies is called a ganglion.

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