THE PROSOMATIC SEGMENTS OF AMMOCGTES 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 paleostraca. 
Yet another clue is afforded by the tubular muscles of Ammoccetes, 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 Ammoceetes 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. 
