O'PHIUROI'DEA (Neo-Lat. nom. pl., from Ok. four, ()phis. serpent apd, our," thin. 'rhe class of brittle stars. or sand stars, of which Ophiura is the typical genus. Ophiuroids (or ophiurans I are star-shaped, freely moving echino Berms (q.v.), with a flat, roundish or polygonal disk, from which suddenly arise live arms, which are slender, cylindrical, and contain no spacious continuation of the etelomie cavity of the disk, or hepatic ewea, while there is no vent. '11w re are no ambulacral grooN es in the arms, and the suckers are but little used in locomotion, whicl is mainly effected by the arms themselves, the 'feet,' or ambulacra, being thrust out laterally and acting as tactile organs. The mouth and also the madreporite are on the under side of the disk. On the ventral surface, also, are five slits which connect with a corresponding number of respiratory saes (bursa.) into which the ovaries or spermaries open. The eggs passing out through these slits are fertilized in the water, the sexes being distinct. The ophiuroids, as a rule, pass through a well-marked metamorphosis, the free-swimming young being called a pluteus. Certain forms undergo self-division, and in others development is direct. The class is divided into
two orders, Ophiurida and Eurya/ida, the lat ter having the arms greatly subdivided into long curly tendrils, as in Astrophyton, the basket fish. Fossil ophiuroids begin to appear in the Silurian period. while genuine modern forms arose in the middle Trias. See BRITTLE STARS.
Both orders of the Ophiuroidea—the Ophiurida having simple arms and the Euryalida with bi-:melted arms—are represented from the Silu rian onward, and the fossil forms show few impor tant differences from their modern descendants. They are usually rare, lint a few localities in the Devonian and in the Triassic and Jurassic shales and limestones have furnished well-preserved specimens in abundance. Such localities are Bundenbach (Devonian) and Solenhofen (Juras sic) in Germany, and Crawfordsville (Carhonifer ons) in Indiana. The more important genera are: Silurian—Eueladia, Protaster, Tieniaster; Devonian—Protaster, Uphiura; Carboniferous Onychaster; Mesozoic—Aspidura, Geocoma, and ophioglypha, with other modern genera. Sec