We describe the external morphology of the two cypridiform larval instars (first and second ascothoracid-larvae, or -a-cyprids") of the ascothoracidan genus Dendrogaster. Ascothoracid-larvae of five species were studied with light and scanning electron microscopy, including both ascothoracid-larval instars in Dendrogaster orientalis Wagin. The first and second instars of the ascothoracid-larvae differ in almost all external features. The carapace of instar 1 has a smooth surface and lacks pores, setae, and lattice organs, while instar 2 has all these structures. The antennules of the first instar have only a rudimentary armament, the labrum does not encircle the maxillae, thoracopods 2-3 are not armed with a plumose coxal seta, and the abdomen is four-segmented (versus five-segmented in instar 2). Thus, the first ascothoracid-larva of Dendrogaster represents a transitional, generally brooded stage between the naupliar stages and the dispersive and fully functional second ascothoracid-larva that accomplishes settlement. The presence of two instars of ascothoracid-larvae (a-cyprids) in members of the order Dendrogastrida differs from the single cypridiform instar found in the Cirripedia (cyprid) and Facetotecta (y-cyprid), and we discuss the evolutionary significance of these ontogenies. We found lattice organs in both the second ascothoracid-larvae and in adult males of Dendrogaster. We could not observe both ascothoracid-larvae and males in any single species, but our data suggests that the lattice organs change significantly at the molt between these two instars. The lattice organs of second ascothoracid-larvae have no distinct keel and are situated in wide, shallow pits, whereas they have the ground pattern "crest-in-a-trough" morphology in adult males of two additional species examined for comparison. The positions of the terminal pores of lattice organs 1 and 2 also seem to change during maturation. These findings show that comparative data on lattice organ morphology for phylogenetic purposes must derive from strictly homologous instars, viz., the second ascothoracid-larva (a-cyprid) of the Ascothoracida, the y-cyprid of the Facetotecta, and the cyprid of Cirripedia. The ascothoracid-larvae of Dendrogaster and those of the family Ascothoracidae have four pairs of lattice organs, which suggests that this genus and family form a monophylum, to the exclusion of Ulophysema, which then brings into question the monophyly of the Dendrogastridae. Ulophysema is currently placed in the Dendrogastridae, but its second ascothoracid-larva has lattice organs of different and more plesiomorphic number and morphology. We briefly review lattice organ morphology across the Thecostraca. These organs are normally considered structures of the cypridiform larva and their presence in adult (males) Ascothoracida is unique in the Thecostraca. The continued morphological modification of these sensory structures in males compared to ascothoracid-larvae may suggest that they originated in adult thecostracans, but have come to be functional in the cypridiform larvae as well. (c) 2008 Elsevier GmbH. All rights reserved