Brittle Stars

Brittle - Stars, the popular name for the "Ophiuroidea," a class of the Echinodermata. This name has been applied to them owing to their habit of breaking off their arms when alarmed. They resemble the Starfish (class Asteroidea), in consisting of a central body, from which radiates a number of arms; but they differ from these (cf. Asterias) in that the arms are sharply marked off from the body, whereas in the Starfish the central disc appears to be formed merely by the fusion of the bases of the arms. The number of these is more constantly five than in the Asteroidea. The structure of the arms is also very different in the two groups; thus in the Brittle-Stars they are more slender, and lack the furrow along the under-side; further, they do not contain any prolongation of the stomach, but are mainly occupied by a row of ossicles or joints. Pairs of small tube feet occur along the under-sides, but locomotion is mainly effected by the use of the arms as limbs. Other differences from the true starfish are the absence of an anus, and the fact that the "madreporite" (the perforated plate which filters the water that enters the water-vascular system) is on the under-side; there may, however, be several of these plates. Further, around the mouth there is a complex arrangement of ossicles which acts as a masticatory apparatus. The reproductive organs are a series of glands discharging their products into chambers around the mouth, known as "genital bursae"; these also serve for respiration. In most cases the larva is a Pluteus (q.v.), a free-swimming form, with a skeleton like an easel. In some cases, however, there is no such metamorphosis, and the young at birth resemble the parents, which are, therefore, viviparous. They have long been known to reproduce by "fission," or the growth of the whole animal from parts. The class "Ophiuroidea" is divided into three orders: the Ophiurida, including the common English forms; the Euryalida, a series with simple or branched, and very flexuous arms; and the Protophiura, an extinct group confined to the Palaeozoic era.