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Tissues - both of which are shown in Figure 10 Both body...

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Tissues: Jellyfish, Corals and Sea Anemones The phylum Cnidaria contains 10,000 species characterized by adult bodies having radial symmetry. Cnidarians are aquatic, mostly all marine. The cnidarian body has only the ectoderm and endoderm tissue layers, making this group diploblastic. Members of this phylum all have stinging cells that eject a barbed thread and possibly a toxin. Only cnidaria have these cnidocytes (shown in Figure 9), a specialized cell that contains a nematocyst, a fluid-filled capsule containing a long, spirally coiled hollow thread. When the trigger of the cnidocyte is touched, the nematocyst is discharged. Some threads merely trap a prey or predator, while others have spines that penetrate and inject paralyzing toxins . These toxins make some jellyfish (and a related group the box jellies) among the most poisonous of animals. Cnidarians have two body forms that may occur: a mobile medusa and a sessile (fancy term for not mobile) polyp
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Unformatted text preview: , both of which are shown in Figure 10. Both body forms have tentacles arranged around an opening into the two-layered sac-like body. The inner tissue layer (derived from endoderm) secretes digestive juices into the gastrovascular cavity, which digests food and circulates nutrients (doing the job our circulatory AND digestive systems do). Muscle fibers occur at the base of the epidermal and gastrodermal cells, making this the first group of muscled animals. Nerve cells located below epidermis near the mesoglea interconnect and form a nerve net throughout the body. Cnidarians have both muscle fibers and nerve fibers, making these animals capable of directional movement. The nerve net allows transmission of messages in more than one direction, possibly an advantage in a radially symmetrical animal, while contraction of muscle fibers (under control of the nerve fibers) allows for movement. While they have a nerve net, brains are not present....
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