1 88 THE ORIGIN OF VERTEBRATES 



cartilaginous bar is here replaced by a bar of muco-cartilage and 

 the branchiae 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 Einne.' 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 Ammocoetes 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 branchiae, 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 branchiae, 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 Palaeostraca was really 

 composed of two pairs of appendages, of which the most posterior 

 carried branchiae, while the anterior pair had amalgamated in the 

 mid-line ventrally, and carried some special organ instead of 

 branchiae, then the accumulation of coincidences is becoming so 

 strong as to amount to proof of the correctness of our line of 

 investigation. 



The First Mesosomatic 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 branchiae, and instead of being 



