Practice question

Holothurians or sea cucumbers possess distinctive internal respiratory structures called respiratory trees, paired highly branched diverticula of cloaca extending anteriorly into coelomic cavity alongside gut. Rhythmic cloacal pumping draws oxygenated seawater into trees where gas exchange occurs across thin epithelium between water and coelomic fluid plus hemal plexus. Trees also aid excretion by accumulating nitrogenous wastes. Absence of dermal papulae due to thick leathery body wall makes tree respiration essential, especially for burrowing species in hypoxic sediments. Ability to eviscerate trees with Cuvierian tubules provides defense, unique among echinoderms.