Morphology of the genital ducts in female crabs

The Brachyura may be separated into two groups by the position of the female genital openings. In the superfamilies Gymnopleura and Dromiacea they open on the coxae of the third peraeopods, while in the Oxystomata, Corystoidea, Brachyrhyncha and Oxyrhyncha they open on the sternum of the corresponding segment. This paper deals with the structure of the genital duets in females of the latter group. Each duct, leading from the ovary to the exterior, consists of four regions—oviduct, spermatheca, vagina and vulva. Two basic patterns of duct, simple and concave, are recognized by variations in the structure of the vagina. Each may be elaborated by the presence of an operculum closing the vulva. These are described by chosen examples, and some of the changes associated with the breeding cycle are detailed. The distribution of the two patterns of duet in the various families of Brachyura is examined, and the phylogenetic implications considered. The more important of these are that the Corystoidea is rightly designated a distinct superfamily, and that the Corystoidea-Portunidae line forms the basic stock of the Brachyura with sternal female openings.