THE PROS O MA TIC SEGMENTS OF AMMOCCETES 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 palasostraca. 



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, wliich 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 Limidus. 



Their nerve-supply in Ammoccetes is most extraordinary ; for, although 

 they are segmentally arranged throughout the whole respiratory region, which 

 is segmentally supplied by the Vllth. IXtli. 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 Vllth, 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. 



