The brain stem consists of the the brain stem consists of the cerebrum, pons, midbrain, and medulla midbrain, medulla, and pons pons, medulla, cerebellum, and midbrain midbrain only

midbrain, medulla, and pons

Explanation:

The brainstem (also known as truncus encephali) is localized in the posterior part of the brain, in the base between cerebral hemispheres and the cervical cord. It's divided into three parts, the midbrain or mesencephalon, the pons or mesencephalon, and the medulla oblongata also known as long medulla or myelencephalon. In the image I added you can see the brainstem structure.

Some of the brainstem functions include the regulation of respiratory and cardiac function, thermoregulation, and other functions related to vision, sleep, etc.

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This relationship is an example of mutualism. Mutualism occurs when two organisms benefit from one another in their existence together. The corals exist on reefs and are near certain types of seaweed that they would need removed. The crabs hide within the corals and actually remove the seaweed for the corals. This demonstrates mutualism as both work together to exist in a mutually beneficial relationship.