The axial skeleton of Malacostraca, because incompletely known, has been little or not taken into account by carcinologists. Nevertheless it encloses many data (or informations) significant as regards to the process of diversification of the families composing this large group. The cephalothoracic metamers of Nephrops norvegicus (Linné, 1758), species selected as a representative example, have been prepared separately and studied one after the other, beginning with the posterior ones and going on with the cephalic ones, in which the modifications become accentuated. A terminology of each sclerite of each metamer (or segment) is stated precisely, founded on their morphologic origin, so as to be able to describe and compare clearly the metamers through the different members (constituants) of superior Crustacea. The study of the spermatheca of the female, an important intersegmentary skeletic structure, the conformation of which was poorly known, allows to understand, in addition to its composition, the one of another segment, the mandiblular one, showing the possibility of scission of the sternal sclerite of a segment, when the sternite of a decapod is generally uneven. In other res-pects, the fact that any sclerite, exists in the mandibular segment, other than what is named «mandibule» regarded as a simple appendage, demonstrates the participation of the whole segment to this mandibule. So the mandibule of decapods, should not be any longer homologated as a simple appendage but be considered as a grinding complex with a perisegmentary articulation, composed of all the sclerites of the metamer fused together, including the regressed appendage.
Crustacea, Malacostraca, Nephrops, segmentation anatomy, axial skeleton, spermatheca, mandibular organ.