This genital chamber, according to Tarnani, opens to the exterior by a single median opening between the operculum and the succeeding segment; similarly, a communication from side to side exists between the second pair of gill-pouches. I have been able to examine _Hypoctonus formosus_ and _Thelyphonus caudatus_, and in both cases, in both male and female, the opening to the exterior of the common chamber for respiration and for the genital products was {208}not a single opening, as described by Tarnani in _Thelyphonus asperatus_, but on each side of the middle line, a round orifice closed by a lid, like the nest of the trapdoor spider, led into the common genital chamber (_Gen. Ch._) into which both uterus and gills opened. In Fig. 77 I have endeavoured to represent the arrangement of the genital and respiratory organs in the male Thelyphonus according to Tarnani's and my own observations. If we may take Thelyphonus as a sample of the arrangement in those scorpions in which the operculum was fused with the first branchial appendage, among which must be included the old sea-scorpions, then it is most significant that their uterus should open internally into a cavity which was continuous with the respiratory cavity. Thus not only the structure of the gland, but also the arrangement of the internal opening into the respiratory, or, as it became later, the pharyngeal cavity, is in accordance with the suggestion that the thyroid of Ammocoetes represents the uterus of the extinct Eurypterus-like ancestor. Into this uterus the products of the generative organs were poured by means of the _vasa deferentia_, so that there was not a single median opening or duct in connection with it, but also two side openings, the terminations of the _vasa deferentia_. These are described by Tarnani in Thelyphonus as opening into the two horns of the uterus, which thus shows its bilateral character, although the body of the organ is median and single; these ducts then pass within the body of the animal, dorsal to the uterus, towards the testes or ovaries as the case may be, organs which are situated in these animals, as in other scorpions, in the abdomen, so that the direction of the ducts from the generative glands to the uterus is headwards. If, however, we examine the condition of affairs in Limulus, we find that the main mass of the generative material is cephalic, forming with the liver that dense glandular mass which is packed round the supra-oesophageal and prosomatic ganglia, and round the stomach and muscles of the head-region. From this cephalic region the duct passes out on each side at the junction of the prosomatic and mesosomatic carapace to open separately on the posterior surface of the operculum, near the middle line, as is indicated in Fig. 75.