Final answer:
Sea cucumbers have a single gonad and exhibit bilateral symmetry as adults. They also have respiratory trees and tentacles around the mouth.
Step-by-step explanation:
Sea cucumbers have a single gonad. They are the only echinoderms that demonstrate bilateral symmetry as adults. Unlike other echinoderms, sea cucumbers have an extended oral-aboral axis which compels them to lie horizontally rather than stand vertically. They also have gill-like structures called respiratory trees and tentacles around the mouth.