The tagmatized echinoderm

Abstract Tagmosis is a derived condition of metamerism in annelids, arthropods, and chordates in which groups of adjacent segments are modified to perform specific functions. Examples are the clitellum of earthworms, the carapace of lobsters, and the sacrum of tetrapods, to mention a few. The argument is advanced here that examples of tagmosis are not hard to find among echinoderms, a fourth major group of metameric animals; but the degree of tagmosis is weak in most cases. The first few segments of the axial skeleton are often conjoined as a perioral tagma (e.g., the jaws and other parts of the oral frame of asteroids and ophiuroids and the lantern supports of echinoids). Several segments beyond the oral frame unite the disc with the base of the arm in ophiuroids. Crinoid pinnules occur in groups along the arms as oral, genital, and feeding pinnules. Porcellanasterid seastars have cribriform organs along the marginal ossicles on the base of the arm for generation of respiratory currents. Well-developed podial pores and podia are restricted to basal segments of the ophiuroid genus Ophiomusium. Ambulacral plates of most post-Paleozoic echinoids occur in clusters that result in arcs of variable numbers of pore pairs and spine tubercles, giving rise to several non-cidaroid tagmatized patterns. The strongest degree of tagmosis occurs in the bilaterally symmetrical echinoids, with petals, phyllodes, plastrons, and frontal ambulacra; and tagmosis differs among the axes. Tagmosis is absent in extant holothuroids, bodies of which are almost entirely composed of extra-axial elements. Tagmosis occurs in the oral regions of many extinct echinoderm classes, and the ambulacra are tagmatized in some, particularly in blastozoan classes and those with branched ambulacra.