STRUCTURE AND GROWTH OF THE CIDAROID SOCKET-JOINT LANTERN OF ARISTOTLE COMPARED TO THE HINGE-JOINT LANTERNS OF NON-CIDAROID REGULAR ECHINOIDS (ECHINODERMATA, ECHINOIDEA)

In regular echinoids the lanterns of Aristotle are gripping lanterns. They are provided, therefore, with strong joints formed by the rotulae and the epiphyses. According to the structure of the joints the author distinguishes between the socket-joint lanterns of Cidaroida and the hinge-joint lanterns of non-cidaroid regular echinoids. The homologies between the joints of both types are discussed in detail. The mobility of the main joints is restricted by secondary joints, and in socket-joint lanterns it is apparently more restricted than in hinge-joint lanterns. The mesodermal endoskeleton of echinoderms is built up of monocrystalline calcitic elements of a spongy nature. The articular surfaces are provided, however, with polycrystalline covers. Polycrystalline calcite is not transformed into a monocrystalline structure by the organism, and therefore the growth of elements which are parts of the joints is controlled by the polycrystalline covers. Differences in the structure of their joints lead to differences in the mode of the growth of the skeletal elements (especially the rotulae), in both hinge-joint lanterns and in socket-joint lanterns. Characteristic of non-cidaroid lanterns are the large foramina and the deep pits of their pyramids. There are strong reasons to suppose that these structures arise by functional adaptation to the forces which are carried over from the epiphyses to the demipyramids in feeding, i.e., due to the inclined hinge-joints these forces act more or less in radially. These forces act, however, tangentially in socket-joint lanterns, which, therefore, have neither foramina nor pits. The author questions the widely held opinion that the cidaroid lantern is the simplest and most primitive stage compared with the lanterns of the recent non-cidaroids. These doubts rest on the paleontological data available. © 1979 Springer-Verlag.