Despite their soft body and slow motion, sea cucumbers present a low predation rate, reflecting the presence of efficient defence systems. For instance, members of the family Holothuriidae rely on Cuvierian tubules for their defence. These tubules are normally stored in the posterior coelomic cavity of the animal, but when the sea cucumber is threatened by a potential predator, they are expelled through the cloacal aperture, elongate, become sticky and entangle and immobilise the predator in a matter of seconds. The mechanical properties (extensibility, tensile strength, stiffness and toughness) of quiescent (i.e. in the body cavity) and elongated (i.e. after expulsion) Cuvierian tubules were investigated in the species Holothuria forskali using traction tests. Important mechanical differences were measured between the two types of tubules, reflecting adaptability to their operating mode: to ease elongation, quiescent tubules present a low resistance to extension, while elongated tubules present a high toughness to resist tractions generated by the predator. We demonstrate that a mutable collagenous tissue (MCT) is involved in the functioning of these organs: (1) some mechanical properties of Cuvierian tubules are modified by incubation in a cell-disrupting solution; (2) the connective tissue layer encloses juxtaligamental-like cells, a cell type present in all MCTs; and (3) tensilin, a MCT stiffening protein, was localised inside these cells. Cuvierian tubules thus appear to enclose a new type of MCT which shows irreversible stiffening.