The crustacean lacinia mobilis: a reconsideration of its origin, function and phylogenetic implications

The lacinia mobilis of the Crustacea Malacostraca is a more or less spine-like movable appendage of the medial mandibular edge, inserted near the base of the incisor process. It occurs in two or possibly three eumalacostracan superorders but is retained in the adult stage only in the Peracarida. The lacinia has been interpreted as the distal member of the spine-row found in many adult Malacostraca and/or their larvae, or alternatively as a derivative of a certain cusp ('cusp b') of the biting edge of the primitive lophogastrids. The distribution, ontogeny and function of the lacinia were studied in a variety of Eumalacostraca. There is great variability in the guiding and locking mechanisms involved in biting, within the subclass and even within single orders. A lacinia-based guiding and locking system is likely to function only in weak mandibles. New evidence is produced in favour of derivation of the lacinia from the spine-row, and the 'cusp b' derivation hypothesis is rejected, 'cusp b' being only a highly specialized lacinia. Doubt is cast upon the unity of the superorder Peracarida mainly because the place of the order Amphipoda within it is regarded as insecure.