The urosome of the Pan- and Peracarida

We have examined the caudal regions of diverse peracarid and pancarid malacostracans using light and scanning electronic microscopy. The traditional view of malacostracan posterior anatomy is not sustainable, viz., that the free telson, when present, bears the anus near the base. The anus either can occupy a terminal, sub-terminal, or mid-ventral position on the telson; or can be located on the sixth pleomere – even when a free telson is present. Furthermore, there is information that might be interpreted to suggest that in some cases a telson can be absent. Embryologic data indicates that the condition of the body terminus in amphipods cannot be easily characterized, though there does appear to be at least a transient seventh segment that seems to fuse with the sixth segment. Our understanding of the genesis of the so-called telson of amphipods could be subject to alternative interpretations than those traditionally put forth.