In strict accordance with the rest of the comparisons made in this region, the pituitary body shows by similarity of structure, as well as of position, that it arose from the coxal glands, which were situated at the base of the four endognaths. {325}One after another, when once the clue has been found, all these mysterious organs of the vertebrate, such as the pituitary and thyroid glands, fall harmoniously into their place as the remnants of corresponding important organs in the palæostraca. Yet another clue is afforded by the tubular muscles of Ammocoetes, that strange set of non-vertebrate striated muscles, which are so markedly arranged in a segmental manner, which disappear at transformation, and are never found in any of the higher vertebrates, for the limits of their distribution correspond to the veno-pericardial muscles of Limulus. Their nerve-supply in Ammocoetes is most extraordinary; for, although they are segmentally arranged throughout the whole respiratory region, which is segmentally supplied by the VIIth, IXth, and Xth nerves, and are found in front of this region only in one segment, that of the lower lip, which is supplied by the velar branch of the Vth nerve, yet they are not supplied segmentally, but only by the velar nerve and a branch of the VIIth, the _ramus branchialis profundus_. This latter nerve extends throughout the respiratory region, and gives off segmental branches to supply these muscles. It is also a curious coincidence that in such a markedly segmented animal as Limulus, a nerve--the pericardial nerve--which arises from the nerves of the chilarial and opercular segments, should pass along the whole respiratory region and give off branches to each mesosomatic segment. It is strange, to say the least of it, that the chilarial or metastomal and the opercular segments of Limulus should, on the theory advocated in this book, correspond to the lower lip and hyoid segments of the vertebrate. At present the homology suggested is not complete, for there is no evidence as yet that the veno-pericardial muscles have anything to do with the pericardial nerve. {326}CHAPTER X _THE RELATIONSHIP OF AMMOCOETES TO THE MOST ANCIENT FISHES--THE OSTRACODERMATA_ The nose of the Osteostraci.--Comparison of head-shield of Ammocoetes and of Cephalaspis.--Ammocoetes the only living representative of these ancient fishes.--Formation of cranium.--Closure of old mouth.--Rohon's primordial cranium.--Primordial cranium of Phrynus and Galeodes.--Summary.