Explain how zooxanthellae have a symbiotic relationship with coral.

There is a symbiotic relationship between algae called zooxanthellae and coral. Coral provides a place to live protecting these microscopic algae from direct ultraviolet light and giving them nutrients such as ammonium and phosphate. Algae provide coral with substances necessary for growth, calcification, nutrition and development, because they transfer the products of photosynthesis.

after receiving a stimus, the sensory receptors send(s) signals to the appropriate area of the body.  

the must be sensory neurons.  

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b. lithosphere, asthenosphere, mesosphere   is the answer most likely