The earliest known crinoids have been found in rocks of Ordovician age, and their remains are particularly abundant in Paleozoic rocks. Crinoids are living today but most of them are stemless free-swimming forms called “feather stars,” much less abundant than their Paleozoic ancestors. Subphylum Eleutherozoa The eleutherozoans are free-swimming, bottom-dwelling, echinoderms which have been divided into two classes. The class Asterozoa (star-shaped echinoderms) contains the subclasses Asteroidea (the starfishes) and the Ophiuroidea (the brittle stars). Although they are known as fossils, neither of these groups is of paleontological importance. The class Echinozoa (echinoderms without laterally directed arm-like extensions) contains the subclasses Echinoidea (the sea urchins and sand dollars) and Holothuroidea (the sea cucumbers). Of these two subclasses, only the Echinoidea are useful fossils. CLASS ASTEROZOA.— These are typical star-shaped free-moving echinoderms in which the body is divided into a central disk and radiating arms. Subclass Asteroidea.— This class contains the starfishes which, although not common fossils, illustrate well the typical echinoderm characteristics (Pl. 35). Fossil starfishes have been found sparingly in certain formations in Texas, but well-preserved specimens are quite rare. However, excellently preserved starfishes have been found in slabs of Cretaceous limestones from central and north-central Texas. Subclass Ophiuroidea.— The ophiuroids are echinoderms with a well-defined central disk and five long, slender, whip-like arms. They have been called brittle stars because of their ability to shed their arms when they are disturbed. Their long, slender, snake-like arms have also resulted in their being called serpent stars. Ophiuroids range from Ordovician to Recent in age, but because of the delicate nature of their bodies they are seldom found as fossils. Ophiuroid remains have been found in certain Mesozoic and Cenozoic rocks of Texas, but they consist largely of small segments of the arms or body fragments. CLASS ECHINOZOA.— The echinozoans are a group of unattached echinoderms whose bodies consist of numerous calcareous plates and spines, but they do not possess the radiating arm-like extensions which characterize the asterozoans. Subclass Echinoidea.— Echinoids are free-moving echinoderms with disk-shaped, heart-shaped, biscuit-shaped, or globular exoskeletons (Pl. 36). Modern representatives of this group include the familiar sea urchins, heart urchins, and the sand dollars. [Illustration: Plate 35] CRINOIDS CRINOID CALYX × ½ CRINOID COLUMNALS × 1 HOLOTHURIAN SCLERITES (GREATLY ENLARGED) CRETACEOUS FOSSIL STARFISHES PENTAGONASTER × 1 PENTACEROS × 1