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The thoracic sternum of the primitive crabs (Podotremata Guinot, 1977) is strongly modified in females at the level of the sutures 7/8, separating the last two sternites, which corresponds to a secondary specialization of the phragmae 7/8. Thus a paired spermatheca has developed, which is intersegmental, internalized and independent of the female gonopores on the coxae of the third pereopods. This is unique to the Podotremata, being completely distinct from the eubrachyuran seminal receptacle. The spermatheca is reviewed in all members of the Podotremata, in its external aspect and internal structure. Among the Dromiacea, a spermathecal tube becomes specialized in the Homolodromiidae, Dromiinae, and Hypoconchinae, while it is absent in the Dynomenidae and Sphaerodromiinae, suggesting that the Sphaerodromiinae are basal to the Hypoconchinae + Dromiinae and that the Dynomenidae are basal to the remaining dromiacean families. The phylogenetic implications are discussed, confirming the distinction of two basal clades, Dromiacea and Homolidea, the peculiar organization found in the Cyclodorippidae, Cymonomidae and Phyllotymolinidae, and the special condition of the Raninoidea. The paired spermatheca proves to be the strongest synapomorphy of the Podotremata, including two Cretaceous families. Hypotheses on female sperm storage and functioning of the spermatheca, on male sperm transfer and the role of gonopods in insemination, and on the modalities of fertilization are included. New data on the axial skeleton are provided. The study of the spermatheca, which has considerable systematic value in decapod phylogeny, leads to a discussion of the monophyly of the Brachyura, taking into account the paleontological data.
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