Abstract
Y-larvae comprise the crustacean taxon Facetotecta. A new kind of ‘nauplius y’, designated Type VI, is described from plankton in Disko Bay. West Greenland. The internal anatomy and some external features of ‘cypris y’ are described from a 2 μm-sectioned specimen collected in Øresund and from a whole-mounted specimen from Disko Bay. Newly discovered or reinterpreted features of ‘cypris y’ include: sessile compound eyes with tripartite crystalline cones; two pairs of eye-associated sensory organs which are homologized with the biramous, plumose cephalic organs found in some Ascothoracida; the antennular segmentation; identification of the ‘oral pyramid’ as the labrum with a posterior mouth and blind pharynx; sparse cephalic musculature except in antennules; first thoracic tergite shared by first two thoracomeres (first thoracomere not fused to head); a complicated array of basal thoracopodal sclerites; a solid oesophagus trace and no differentiated midgut (dorsal yolk or oil globules instead); a condensed nervous system; trunk and limb musculature. A cladistic analysis is conducted of the Facetotecta and the other maxillopodan groups previously linked to them by various authors (Ascothoracida. Cirripedia, Branchiura. Copepoda). The last two groups listed are the more plesiomorphic. and among the other three an Ascothoracida-Cirripedia or Facetotecta-Cirripedia sister-group relationship is most likely, with the former slightly more parsimonious. The Facetotecta, Ascothoracida, and Cirripedia are reclassified as superorders within the resurrected maxillopodan subclass Thecostraca GRUVEL.
