202 THE O RIG IX 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 iKiloco-liysieroiii, and was 

 derived from the uterus of the ancient pala^ostracan 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 



