On the Significance of Larval and Juvenile Morphology for Suggesting Phylogenetic Relationships of the Vestimentifera

Abstract

Synopsis. Examination of larvae and juveniles ofRidgeia has revealed the presence of a prototroch, metatroch and neurotroch. A complete diges? tive tract is present in juveniles, the esophageal portion of which passes through the brain. The major longitudinal, pulsatile blood vessel is on the side opposite the nerve cord and, along with the longitudinal blood vessel adjacent to the nerve cord, is situated in the medial mesentery separating lateral coelomic cavities. The posterior body region is multisegmented, and the lateral cavities of each segment are separated by a medial mesentery. Newly forming segments exhibit paired spaces that expand to form coelomic cavities of that segment. All of these characters are held in common with the Annelida and strongly suggest that the nerve cord is ventral and the major pulsatile blood vessel is dorsal; the brain is, in effect, a circumesophageal ganglion; and coelom formation is by schizocoely and mesodermal origin is teloblastic. It is suggested that the Ves? timentifera are allied with the gastroneuralians and may well have arisen near the base of the phylogenetic line leading to the Annelida. Avenues of future research, likely to shed more light on the position of the Ves? timentifera in invertebrate phylogeny, are suggested.