Final answer:
Sea cucumbers perform gas exchange through structures known as respiratory trees, where water is actively pumped in and out by muscles around the cloaca.
Step-by-step explanation:
Gas exchange in sea cucumbers takes place in the respiratory trees, which are a pair of gill-like structures that branch from the posterior part of the digestive tract. Muscles around the cloaca actively pump water in and out of these respiratory trees, facilitating the exchange of gases. Sea cucumbers lie horizontally due to their extended oral-aboral axis and display a form of "functional" bilateral symmetry. Unlike other echinoderms, some sea cucumber species have cells containing hemoglobin, which is unique to their class Holothuroidea. Their feeding mechanisms vary; some sea cucumbers are detritivores consuming detritus, while others are suspension feeders, using their tentacles to sift out tiny organisms from the water.