Ophiuroidea possess a skeleton of calcium carbonate in the form of calcite. In ophiuroids, the calcite ossicles are fused to form armor plates which are known collectively as the test. The plates are covered by the epidermis, which consists of a smooth syncytium. In most species, the joints between the ossicles and superficial plates allow the arm to bend to the side, but cannot bend upwards. However, in the basket stars, the arms are flexible in all directions. Both the Ophiurida and Euryalida (the basket stars) have five long, slender, flexible, whip-like arms, up to 60 cm in length. They are supported by an internal skeleton of calcium carbonate plates referred to as vertebral ossicles. These "vertebrae" articulate by means of ball-and-socket joints, and are controlled by muscles. They are essentially fused plates which correspond to the parallel ambulacral plates in sea stars and five Paleozoic families of ophiuroids. In modern forms, the vertebrae occur along the median of the arm. The ossicles are surrounded by a relatively thin ring of soft tissue, and then by four series of jointed plates, one each on the upper, lower, and lateral surfaces of the arm. The two lateral plates often have a number of elongated spines projecting outwards; these help to provide traction against the substrate while the animal is moving. The spines, in ophiuroids, compose a rigid border to the arm edges, whereas in euryalids they are transformed into downward-facing clubs or hooklets. Euryalids are similar to ophiurids, if larger, but their arms are forked and branched. Ophiuroid podia generally function as sensory organs. They are not usually used for feeding, as in Asteroidea. In the Paleozoic era, brittle stars had open ambulacral grooves, but in modern forms, these are turned inward. In living ophiuroids, the vertebrae are linked by well-structured longitudinal muscles. Ophiuroida move horizontally, and Euryalina species move vertically. The latter have bigger vertebrae and smaller muscles. They are less spasmodic, but can coil their arms around objects, holding on even after death. These movement patterns are distinct to the taxa, separating them. Ophiuroida move quickly when disturbed. One arm presses ahead, whereas the other four act as two pairs of opposite levers, thrusting the body in a series of rapid jerks. Although adults do not use their tube feet for locomotion, very young stages use them as stilts and even serve as an adhesive structure. Reproduction The sexes