Ophiuroids have long slender flexible arms that are sharply separated from the disk. The common name "brittle star" refers to the fact that the arms of many species are easily broken off. Locomotion involves the entire arm; movement is made possible by an internal skeleton that supports the arm. The skeletal portions of the arm are made of calcium carbonate and look like vertebrae; they are called vertebral ossi cles. Five arms are usual, but a few species of ophiurid ophi-uroids have six or seven, and euryalid ophiuroids may have as many as 20 major additional arms.
Species with branched arms are restricted to the families Gorgonocephalidae and Euryalidae and are called basket stars. The basket stars are the largest ophiuroids; Gorgonocephalus stimpsoni can measure up to 27.5 in (70 cm) in arm length with a disk diameter of 5.63 in (14.3 cm). The branching of the arms in basket stars is repeated in the formation of smaller and smaller units in a fernlike manner.
The upper surface of the disk in ophiurid ophiuroids is covered with a series of scales. In contrast, euryalid ophiuroids are characterized by the presence of a covering on the disk without any large plates underneath. The mouth on the lower surface of the disk is framed by five jaws bearing spinelike teeth and papillae. The mouth leads directly to the saclike stomach, which ends in a blind pouch since the organism lacks intestines and an anus.
The oral side of the disk contains the bursal slits on each side of the base of each arm. These slits are openings for the respiratory bursae, which are specialized sac- or pouchlike formations in the body wall that serve for gas exchange. The go-nads are attached to the walls of the bursae; in a few species, however, the gonads extend into the coelom (body cavity) of the arms. The number of gonads varies.
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