202 THE ORIGIN OF VERTEBRATES 
pits, as determined by Miss Alcock. Although there is no indication 
on the ventral surface of the skin of any difference between the 
anterior and posterior portions of the respiratory region, yet when 
the ventral rows of the epithelial pits supplied by each branchial 
nerve are mapped out, we see how the most anterior ones diverge 
more and more from the mid-ventral line, following out exactly the 
limits of the underlying muco-cartilaginous thyroid plate (Fig. 84). 
The whole evidence strongly leads to the conclusion that the 
thyroid portion of the facial segment was inserted as a median tongue 
between the foremost branchial segments on each side, and that, 
therefore, the whole facial segment, consisting as it does of a thyroid 
part and a hyoid or branchial part, may be represented as in Fig. 
85, which is obtained by splitting an Ammoccetes longitudinally 
along the mid-dorsal line, so as to open out the pharyngeal chamber 
and expose the whole internal surface. The facial segment is marked 
out by shading lines, the glosso-pharyngeal and vagus segments and 
the last of the trigeminal segments being indicated faintly. The 
position of the thyroid gland is indicated by oblique lines, C being 
the curled portion. 
THE UTERUS OF THE SCORPION GROUP. 
Seeing how striking is the arrangement and the structure of the 
glandular tissue of this thyroid, how large the organ is and how 
absolutely it is confined to Ammoccetes, disappearing entirely as 
such at transformation, we may feel perfectly certain that a corre- 
sponding, probably very similar, organ existed in the invertebrate 
ancestor of the vertebrate; for the transformation process consists 
essentially of the discarding of invertebrate characteristics and the 
putting on of more vertebrate characters; also, so elaborate an organ 
cannot possibly have been evolved as a larval adaptation during the 
life of Ammoccetes. We may therefore assert with considerable con- 
fidence that the thyroid gland was the paleo-hysteron, and was 
derived from the uterus of the ancient palzostracan forms. If, then, 
it be found that a glandular organ of this very peculiar structure and 
arrangement is characteristic of the uterus of any living member of 
the scorpion group, then the confidence of this assertion is greatly 
increased. 
In Limulus, as already stated, the genital ducts open separately 
