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me to be the metamerical arrangement of the head nerves. 
If FRORIEP’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 neuromerism 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 die Branchio- 
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 FRORIEP'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nerves, after mesomerism had been replaced by branchio- 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 
reterence 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. ZIEGLER (1915 p. 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 ZIEGLER's suggestion, 
their number might appear to be still greater. Of these 
