Sea cucumbers are the least radially symmetrical group but the most advanced in terms of the complexity of their organs. Holuthuroids may be thought of as elongated sea urchins, lying on one side. They lack a fused skeleton and have only scattered, reduced ossicles in their tough skins. They deter predators by ejecting sticky threads when attacked. Their large buccal podia are used to trap particulate material, either from the water or by sweeping over the substrate. Food is wiped off the tentacles by stuffing them in and out of the pharynx, one at a time. Some sea cucumbers move either by means of their podia, which may be greatly reduced or by contractions of longitudinal and circular muscles in the body wall like earthworms.
A strange relationships exists between certain sea cucumbers and slender Pearlfish which shelter in the cucumber's respiratory trees when not foraging for food.