208 THE ORIGIN OF VERTEBRATES 
not a single opening, as described by Tarnani in Thelyphonus aspe- 
ratus, 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, isin accordance with the suggestion that the thyroid of Ammo- 
ecetes 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- 
cesophageal 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 meso- 
somatic carapace to open separately on the posterior surface of the 
operculum, near the middle line, as is indicated in Fig. 75. 
We have, therefore, two distinct possible positions for the genital 
ducts among the group of extinct scorpion-like animals, the one 
from the cephalic region to the operculum, and the other from the 
abdominal region to the operculum. 
