192 THE ORIGIN OF VERTEBRATES 
from the internal surface, might be represented by Fig. 79, in which 
the thick line represents the outline of the opercular segment, and 
the fainter lines the succeeding 
en, duct Ut. Mase. Gen. duct. branchial segments. The middle 
and anterior part of the opercular 
segment carried the terminations 
of the genital organs; these I 
have represented, in accordance 
with our knowledge of the nature 
of these organs in the present-day 
scorpions, as a median elongated 
uterus, bilaterally formed, from 
which the genital ducts passed, 
probably as in Limulus, towards 
a mass of generative gland in the 
Fic. 79.—D1aGRAM TO INDICATE THE cephalic region, and not as in 
ccs chs aah a Scorpio or Thelyphonus, tailwards 
MATIC SEGMENTS OF HURYPTERUS. 2 
The opercular segment is marked out by to the abdominal region. 
the thick black line. The segments It is possible that in Holm’s 
I-VI. bear branchie, and segment I. representation of Eurypterus, Fig. 
is supposed in the male to carry the 104, the genital duct on each side 
uterus masculinus (Ut. Masc.) and 9? ” 
the genital ducts. ; is indicated. . 
THE THYROID GLAND oF AMMOCGTES. 
If we compare this mesosomatic region of Eurypterus with that 
of Ammoccetes, the resemblance is most striking, and gives a mean- 
ing to the facial nerve which is in absolute accordance with the 
interpretation already given of the glossopharyngeal and vagus 
nerves. In both cases the foremost respiratory or mesosomatic 
segment is double, the posterior lateral part alone bearing the 
branchie, while the median and anterior part bore in the one animal 
the uterus and genital ducts, in the other the thyroid gland and 
ciliated grooves. We are driven, therefore, to the conclusion that 
this extraordinary and unique organ, the so-called thyroid gland of 
Ammocetes, which exists only in the larval condition and is got rid 
of as soon as the adult sexual organs are formed, shows the very form 
and position of the uterus of this invertebrate ancestor of Ammo- 
ecetes. What, then, is the nature of the thyroid gland in Ammocceetes ? 
