The wide-set eyes give hammerheads long visual range, better than most sharks.

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Scalloped Hammerhead Shark

Dolphins use echolocation to "see" the environment, similar to a sonar or radar. They send out bursts of clicking sounds, and based on the frequency of the return signal, and how much time the signal used to return, the dolphin can estimate what kind of objects are nearby.

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Spinner Dolphin

Nudibranchs have shells as larva, but shed them when they mature. Shells are heavy and make moving around difficult. Most molluscs use the shell as a primary defence, but nudibranchs have developed other techniques for protection, such as toxic secretion. A few make their own poisons, but most utilize the toxic metabolites from what they eat.