80 THE ANCESTRY OF VERTEBRATES 



me to be the metamerical arrangement of the head nerves. 

 If FRORlEP's views were right, we ought to assume that, 

 simultaneously with the dissolution of the original head 

 somites, the spinal nerves which originally belonged to 

 them had disappeared also, and that afterwards the influence 

 of the branchiomerism had caused the production of a 

 quite new series of nerves, corresponding in their arrange- 

 ment to the gill-slits but at the same time showing 

 unmistakable traces of resemblance to the spinal nerves 

 of the trunk. The neuromierism is considered as of quite 

 secondary significance and as immediately dependent upon 

 meso- or branchiomerism. As Gegenbaur (1887, p. 93) 

 observes in a polemic against AHLBORN: "Um dieBranchio- 

 merie zu retten, stellt er die Bedeutung der Nerven 

 in Abrede, auch ihren metameren Charakter." It seems 

 hardly doubtful, that in phylogeny the neuromerism has 

 been engendered by the myomerism, but it is less probable 

 that this primary neuromerism, once established, would 

 so easily give way to a secondary neuromerism, caused 

 by the branchiomerism. We might rather expect, placing 

 ourselves on FRORlEP's standpoint, that the original head- 

 nerves, after mesomerism had been replaced by branchio- 

 merism, had inclined to adapt themselves to the new con- 

 ditions, and that in the new arrangement, resulting in this 

 way, we should be able to find traces of the old. If then 

 branchiomerism did not correspond to the original meso- 

 merism, we might expect to find e.g. two or three segmental 

 nerves in some gill-bars, or on the contrary, none at all, 

 or one segmental nerve sending branches to two or three 

 gill-bars. But applying this principle to the facts, we can 

 only come to the conclusion that, on the whole, the branch- 

 iomerism corresponds to the mesomerism. Only with 

 reference to the posterior gill-slits, innervated by the vagus, 

 is there room for some doubt, since here indeed we have 

 the case, that more than one gill-slit is supplied by the 

 branches of a single nerve. ZlEGLER (1915p. 462) assumes 

 that originally there were only four post-spiracular gill-slits, 

 situated intersegmentally, while the last gill-slit in pentanch 

 and the last two or three in hexanch and heptanch Elas- 

 mobranchs are phylogenetically younger, and if VAN 

 Wyhe's and HATSCHEK's view of the double nature of 

 the vagus were combined with ZlEGLER's suggestion, 

 their number might appear to be still greater. Of these 



