The significance of moulting in Ecdysozoan evolution

Three major bilaterian clades first appear in the Early Cambrian fossil rec ord: Deuterostomia, Lophotrochozoa, and Ecdysozoa. The taxa placed in Ecdys ozoa are characterized by a moulting habit, unknown in the other major clad es. The origin and consequences of moulting are of fundamental importance t o the history of the ecdysozoan clade, chiefly because moulting precludes m otile ectodermal cilia. Moulting may have originated as an adaptation to pe rmit the enlargement, during growth, of secreted cuticular spines, flanges, and other structures used as ancillary locomotory devices. A combination o f phylogenetic and fossil evidence suggests that the early members of these clades were small vermiform paracoelomates that likely lacked indirect-dev eloping planktotrophic larvae. Thus, the evolution of planktotrophic larvae may have been independently achieved at least three times within Bilateria . The nonmoulting clades evolved larvae that swim and feed via ciliated tuf ts and bands, presumably intercalating these forms within their early devel opmental systems. Within Ecdysozoa, feeding larvae lacked ciliary feeding t racts and evolved by modification of early instars, employing limbs or seta e to generate feeding currents. The setting aside during larval life of cel ls that give rise to adult features is probably an adaptation associated wi th metamorphosis.