188 THE ORIGIN OF VERTEBRATES 
cartilaginous bar is here replaced by a bar of muco-cartilage and 
the branchie are confined to the posterior part of each appendage. 
The anterior portion is, as is seen in Fig. 74, largely occupied by 
blood-spaces, but in addition carries the ciliated groove (ps. br.) called 
by Dohrn ‘pseudo-branchiale Rinne.’ This groove leads directly 
into the thyroid gland, which is a large bilateral organ situated in 
the middle line, as seen in Fig. 80 and Fig. 85. As shown by Miss 
Alcock, the facial nerve supplies this thyroid gland, as well as the 
posterior hyoid branchial segment, and, as pointed out by Dohrn, 
there is every reason to consider this thyroid gland as indicative of 
a separate segment, especially when van Wijhe’s statement that the 
hyoid segment is in reality double is taken into account. 
The evidence, then, of Ammoccetes points directly to this con- 
clusion: The facial nerves represent the foremost of the mesoso- 
matic group of nerves, and supply two segments, which have amalga- 
mated with each other. The most posterior of these, the hyoid 
segment, is a branchial segment of the same character as those 
supplied by the vagus and glossopharyngeal nerves; represents, 
therefore, the foremost pair of branchial appendages. The anterior or 
thyroid segment, on the other -hand, differs from the rest in that, 
instead of branchie, it carries the thyroid gland with its two ciliated 
grooves. If this segment, which is the foremost of the mesosomatic 
segments, also indicates a pair of appendages which carry the thyroid 
gland instead of branchize, then it follows that this pair of appendages 
has joined together in the mid-line ventrally and thus formed a 
single median organ—the thyroid gland. If, then, we find that the 
foremost of the mesosomatic appendages in the Palostraca was really 
composed of two pairs of appendages, of which the most posterior 
carried branchiz, while the anterior pair had amalgamated in the 
mid-line ventrally, and carried some special organ instead of 
branchie, then the accumulation of coincidences is becoming so 
strong as to amount to proof of the correctness of our line of 
investigation. 
THe First MrEsosoMAtic SEGMENT IN LIMULUS AND ITS ALLIES. 
What, then, is the nature of the foremost pair of mesosomatic 
appendages in Limulus. They differ from the rest of the mesosomatic 
appendages in that they do not carry branchie, and instead of -being 
