“…Although there are now some data on devel- opmental roles of RA signaling emerging from invertebrate chordates, there is still an obvious lack of information on the role(s) of RA in non-chordates. Even if low concentrations of 9-cis and all-trans RA have been observed in regenerating limb blastemas of the crab Uca pugilator (Hopkins, 2001) and effects of treatments with RA agonists or antagonists have been described in sea urchins (Sciarrino and Matranga, 1995;Kuno et al, 1999), mollusks (Créton et al, 1993) crustaceans (Chung et al, 1998;Hopkins, 2001;Söderhäll et al, 2006), insects (Picking et al, 1996;Shim et al, 1997;Sun et al, 1993), planarians (Romero and Bueno, 2001), cnidarians (Müller, 1984), and sponges (Imsiecke et al, 1994;Nikko et al, 2001;Wiens et al, 2003), the actual presence and putative roles of RA signaling during development in these taxa remain elusive. Thus, to fully understand the origin and evolution of RA signaling during embryonic development, we need to broaden the sampling of animal taxa and apply more sophisticated experimental tools to non-vertebrate model systems.…”