ABSTRACT
I. INTRODUCTION Ascidians are sessile, filter-feeding marine animals. They were considered mollusks until the second half of the nineteenth century, but this view changed radically when Kowalevsky recognized that the larva had a typically vertebratelike body plan (1). Vertebrate features are apparent in the dorsal tubular nervous system, ventral gut, and the axial notochord with its flanking rows of muscle cells. Additional chordate affinities are also observed during embryonic development; these include the origin of the mouth (pharynx) as a secondary opening in the region of the embryo opposite the blastopore and formation of the neural tube by rolling up of a dorsal neural plate. Today ascidians are classified in the subphylum Urochordata (class Ascidiacea) of the phylum Chordata. This subphylum contains two other classes, the Larvacea and the Thaliacea, both of which are highly specialized for permanent planktonic existence (2). These two groups are much less successful than the ascidians in terms of species diversity.
