Contributions to Zoology, 75 (1/2) 1-21 (2006) The urosome of the Pan- and Peracarida Franziska Knopf1, Stefan Koenemann2, Frederick R. Schram3, Carsten Wolff1 (authors in alphabetical order) 1Institute of Biology, Section Comparative Zoology, Humboldt University, Philippstrasse 13, 10115 Berlin, Germany, e-mail:
[email protected]; 2Institute for Animal Ecology and Cell Biology, University of Veterinary Medicine Hannover, Buenteweg 17d, D-30559 Hannover, Germany; 3Dept. of Biology, University of Washington, Seattle WA 98195, USA. Key words: anus, Pancarida, Peracarida, pleomeres, proctodaeum, teloblasts, telson, urosome Abstract Introduction We have examined the caudal regions of diverse peracarid and The variation encountered in the caudal tagma, or pancarid malacostracans using light and scanning electronic posterior-most body region, within crustaceans is microscopy. The traditional view of malacostracan posterior striking such that Makarov (1978), so taken by it, anatomy is not sustainable, viz., that the free telson, when present, bears the anus near the base. The anus either can oc- suggested that this region be given its own descrip- cupy a terminal, sub-terminal, or mid-ventral position on the tor, the urosome. In the classic interpretation, the telson; or can be located on the sixth pleomere – even when a so-called telson of arthropods is homologized with free telson is present. Furthermore, there is information that the last body unit in Annelida, the pygidium (West- might be interpreted to suggest that in some cases a telson can heide and Rieger, 1996; Grüner, 1993; Hennig, 1986). be absent. Embryologic data indicates that the condition of the body terminus in amphipods cannot be easily characterized, Within that view, the telson and pygidium are said though there does appear to be at least a transient seventh seg- to not be true segments because both structures sup- ment that seems to fuse with the sixth segment.