Sponges (Phylum Porifera) are the simplest animals
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sessile animals that suspension feed on bacteria using specialized cells called choanocytes -
distinguishing characters: one to two cell layers with no true organs, generally asymmetrical, acoelomate -
structure: incurrent ostia, filtering chamber, excurrent osculum -
sponges are important economically for chemical prospecting, ecologically for water filtration
Jellyfish, corals, anemones, and hydroids (Phylum Cnidaria)
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distinguishing characters: radially symmetrical, acoelomate animals with only two (true) tissue layers. -
many species with two body forms to life cycle: the medusa and polyp, many reproduce asexually as well as sexually -
all posess specialized stinging cells called cnidocytes -
cnidarian guts have only one opening that leads to a gastrovascular cavity, which functions for digestion, circulation and support -
cnidarians are important medically for some stings, ecologically as fish predators, or coral reefs
Flatworms (Phylum Platyhelminthes)
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simplest bilaterally symmetrical animals with organ system level of organization -
includes free-living groups and two parasitic groups (flukes and tapeworms); parasites typically have attachment structures (e.g. a scolex), larger reproductive systems, and complex life cycles -
characters: three tissue layers, flat body to allow transport by diffusion, incomplete digestive tract (gastrovascular cavity), simple nervous system, acoelomate
Roundworms (Phylum Nematoda)
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abundant, simple, pseudocoelomate worms -
includes free-living and parasitic forms, some important human parasites (Trichinella, pinworms), agricultural pests and study organisms for genetics -
characters: three tissue layers with organ systems, exoskeleton/cuticle that they must molt, flow-through gut with specialized regions, often fixed number of cells in adult
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