Animals are multicellular heterotrophs that feed by ingestion.
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animals are multicellular heterotrophs that feed by ingestion -
molecular and morphological evidence suggests they probably evolved from colonial protists called choanoflagellates -
animals lack cell walls; instead, cells are held together by extracellular proteins and special intracellular junctions -
most animals have muscle and nerve cells for sensation and coordinated movement
Development typically proceeds through several stages:
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fertilization and formation of the zygote -
mitotic division to form a hollow ball of cells (the blastula) -
infolding of one side of the blastula to form the gastrula. - Taxa in which this opening (blastopore) becomes the mouth are called protostomes;
- Taxa in which this opening becomes the anus are called deuterostomes.
- These groups also have distinctive patterns of early cell division and coelom formation
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differentiation into tissue layers (see below). -
some groups develop directly into adults, while others first form a larva, which then undergoes a major change in body form (metamorphosis) to become the adult.
Embryonic tissue layers give rise to specific tissues.
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ectoderm becomes the epidermis and nervous system -
endoderm becomes the digestive tract and associated organs -
mesoderm, where present, forms muscle, connective tissue, skeleton, circulatory system
Levels of organization include:
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cellular: where different cell types are specialized to particular functions -
tissue: where cells of one or several types work together for common function, and are bound together with specialized junctions and basal lamina -
organ: where different tissues are bound together in a structure for one purpose -
organ system: where multiple organs coordinate for a particular function
Symmetry
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Symmetry is how an organism may be bisected to form pieces that are mirror images of each other. -
asymmetrical organisms have amorphous forms that cannot be equally bisected -
radially symmetrical organisms can be divided in any plane about at least one axis; these organisms have no "head", adapted to sedentary/planktonic life -
bilaterally symmetrical organisms can only be divided in one plane; such organisms have a "head" which is specialized for sensing the environment
Body spaces
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An internal body space forms a mechanical barrier between endoderm and ectoderm, allows for the development and expansion of new structures, serves as a storage chamber for body products (e.g. gametes), provides a circulation medium, and serves as a hydrostatic skeleton -
a true coelom is a fluid-filled space completely lined by mesoderm -
a pseudocoelom is only lined with mesoderm on one side (usually the outside), and is most common in small animals -
acoelomates have no internal body space, and must have spongy mesoderm that allows circulation
Entire taxa are becoming threatened as invasive species spread to new environments
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