Diplospondylia

(from Greek diploos, “double,” and spondylos, “vertebra”), the development in lower vertebrate animals of two complexes for each vertebra—anterior and posterior arches or bodies. There are two kinds of diplospondylia: primary and central. Primary diplospondylia (the formation in each segment of the body of two pairs of anterior and posterior arches) is characteristic of adult members of the Agnatha and of cartilaginous fish. (However, in the embryonic stage, it is characteristic of all fish and amphibians.) Central diplospondylia (the development of two vertebral bodies in each segment), which is a consequence of primary diplospondylia (since formation of vertebral bodies occurs through the merging of several centers that are in their embryonic stage derived from the precursors of the arches), is characteristic of many cartilaginous and bony fish and some Stegocephalia.