What defensive organ can some sea cucumbers eject when attacked?

Some sea cucumbers can eject Cuvierian tubules as a defensive response to attack.

Cuvierian tubules are fine, sticky structures associated with the respiratory tree in certain sea cucumbers, especially members of the genus Holothuria. When threatened, an animal may release them through the anus. In seawater, the tubules can rapidly elongate and become adhesive, entangling or discouraging a predator.

The response is an example of autotomy-related defense: a sea cucumber can sacrifice part of its body and later regenerate the lost structures. Some species also expel internal organs in a broader defensive behavior called evisceration, then regrow them over time.

Not all sea cucumbers possess Cuvierian tubules, and the structures should not be confused with the respiratory trees themselves. Sea cucumbers are echinoderms, relatives of starfish and sea urchins, but their elongated bodies and soft exterior make their defensive biology especially unusual.