The =generative organs= are more or less embedded in the gastric glands. There are two testes, each composed of a pair of intercommunicating tubules, and connected by a common vas deferens with the generative aperture, which is furnished with a double protrusible intromittent organ. A pair of vesiculae seminales and a pair of accessory glands are also present. The female possesses a single ovary, consisting of a median and two lateral tubules, all connected by cross branches. In addition to the external sclerites a free =internal skeletal plate=, situated in the prosoma between the alimentary canal and the nerve-cord, furnishes convenient fulcra for muscular attachment. It is known as the “endosternite.” Brauer[246] has made the most complete study of the development of _Scorpio_, and two of the most interesting of his conclusions may be mentioned here. He has shown the lung-books to be derived from gills borne on mesosomatic appendages. Moreover he found in the embryo five pairs of segmental ducts—in segments 3–6 and 8—and demonstrated that those of segment 5 persisted, though without external aperture, as coxal glands, and those of segment 8 as the genital ducts. =Classification.= More than 350 species of scorpions have been described, but many of these are “doubtful,” and probably the number of known forms may be put at about 300. These are divided by Kraepelin[247] into six families and fifty-six genera. The best indications of the family of a scorpion are to be found in the shape of the sternum, the armature of the tarsi, and the number of the lateral eyes, while assistance is also to be derived from the shape of the stigmata and of the pectines, and from the absence or presence of a spine beneath the aculeus. The six families are: Buthidae, Scorpionidae, Chaerilidae, Chactidae, Vejovidae, and Bothriuridae. =Fam. 1. Buthidae.=—_Sternum small and generally triangular. Tibial spurs in the third and fourth legs. Generally a spur beneath the aculeus. Lateral eyes three to five in number._ There are two sub-families: BUTHINAE and CENTRURINAE. The BUTHINAE, which possess a tibial spur, comprise fourteen genera, most of them Old World forms. The principal genera are _Buthus_, which contains about 25 species, and _Archisometrus_ with 20 species. One genus only, _Ananteris_, is South American, and it includes only a single species. The genus _Uroplectes_, with 16 species, is almost entirely African. The CENTRURINAE, without tibial spur, are New World scorpions, though _Isometrus europaeus_ (_maculatus_) is cosmopolitan. The principal genera are _Tityus_ with 30 species, _Centrurus_ with 13, and _Isometrus_ with 6. =Fam. 2. Scorpionidae.=—_Sternum broad and pentagonal, with sides approximately parallel. No tibial spur, but a single pedal spur. Generally three lateral eyes._