Echinodermata

Pawson, David L., 2007, Phylum Echinodermata *, Zootaxa 1668, pp. 749-764 : 750

Echinodermata as Deuterostomes

The extant Deuterostomia (“second mouth”) are usually defined as animals in which the mouth develops from a second opening in the embryo, opposite to the initial opening, the blastopore, of the rudimentary gut. In addition the coelom develops by enterocoely, or pouching from the primitive gut. Smith (2004b) noted that there was fossil evidence to show that the major deuterostome groups were established by about 520 million years ago (see below under “Other extinct classes of echinoderms”). Composition of the extant Deuterostomia has changed in recent years. At present, it is usually regarded as comprising the phyla Chordata, Hemichordata, and Echinodermata ( Cameron et al., 2000) , as well as the recently-defined phylum Xenoturbella ( Bourlat et al., 2006). In echinoderms with planktotrophic larval stages, the deuterostome affinities of the group are evident. Fell (1948) and others have pointed out that in a significant percentage of echinoderms 1) the coelom develops from a splitting in mesoderm and not from pouching, and 2) the larval mouth becomes the adult mouth. Pawson and Kerr (2001) reported presence of chitin in one species of sea cucumber. Chitin is usually unknown in deuterostomes, but it has been reported from a blenniid fish ( Wagner et al., 1993). These various exceptions to the deuterostome “norm” are believed to be relatively minor, and the echinoderms are regarded as fully qualified members of the Deuterostomia.