Notomithrax ursus actively cuts and attaches adventitious materials to the hooked hairs on its dorsal exoskeleton. Investigation of the general biology, ecology, and behaviour of N. ursus (Section I) showed that these crabs invest considerable amounts of time and energy, at all stages of their life history, in forming and maintaining the mask. This is consistent with masking being an important adaptation. Data from the field and laboratory were consistent with the hypothesis that the mask functions to render the crabs eucryptic. N. ursus were anachoretic most of the time, remaining closely associated with algal clumps. These crabs were nocturnal and spaced out when active. They exhibited large scale dispersal but movement occurred under cover concealment. In addition, N. ursus emphasised behaviours conducive to concealment. When possible, they remained immobile. When active, movement was slow and inconspicuous. Motion was sometimes accompanied by rocking movements. The data indicated that the mask made the crab procryptic not anti cryptic. Preliminary experiments suggested that octopuses were important natural predators of N. ursus. Systematic laboratory observations of the interaction between free-ranging N. ursus and octopuses demonstrated that the algal mask impaired the triggering of attack by the octopuses in response to distal cues. Experiments with differing backgrounds confirmed that the mask most likely functions as eucrypsis rather than special protective resemblance. In addition, a match between the mask and the background, spacing out between individuals, and immobility of the crabs all enhanced the protection of N. ursus. In an experiment in which Octopus maorum was constrained to hunt by visual cues, significantly more unmasked than masked N. ursus were attacked in paired presentations against a matching background. Selection indices ranging from 0.63 - 0.76 were calculated. The eucryptic effect of the mask was dependent on ambient light intensity. It is argued that this is a phenomenon of general significance in the evolution of eucrypsis in animals.