Lattice organs in y-cyprids of the Facetotecta and their significance in the phylogeny of the Crustacea Thecostraca

Scanning and transmission electron microscopy (SEM and TEM) were used to study lattice organs in facetotectan y‐cyprids from the White Sea and from Norwegian and Bahamian waters. The larvae represent at least four and possibly five different species of Facetotecta. Y‐cyprids have five pairs of lattice organs in the head shield (carapace) organized into two anterior pairs and three posterior pairs. Both groups of lattice organs are arranged around a large central pore. The facetotectan lattice organs are elongate areas with a longitudinal keel, just as in the Ascothoracida and some Cirripedia Acrothoracica. The terminal pore of the organs is situated posteriorly in all five pairs. TEM confirms that the organs have the same general morphology as in the Cirripedia and Ascothoracida, namely, a cuticular chamber into which project ciliary segments from the chemosensory cells. Unlike Cirripedia the cuticular roof of the chamber lacks any pores. We conclude that five pairs of lattice organs represent an autapomorphy for the Thecostraca, which supports the monophyly of this taxon. In the ground pattern the terminal pore is posterior in all five pairs. The anterior position of the pore in lattice organ pair 2 is apomorphic for the Cirripedia, while within this taxon an anterior position also in pair 1 is apomorphic for a monophylum comprising the Thoracica and the Rhizocephala. Minute pores in the roof of the organs is another apomorphy of the Cirripedia, but its elaboration into pores visible with SEM may have been subject to some homoplasy. Since lattice organs are omnipresent in the settling instar of the Thecostraca they probably serve a critical role for the function of these cypris or cypris‐like larvae.