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, is in accordance with the suggestion that the thyroid of Ammo- 

 ccetes 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 - 

 (esophageal 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 nieso- 

 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. 



