196 J. B. Johnston 
of myotomes contribute to these muscles, since the last gill slit is 
somewhat farther back, and nine ventral nerves beginning with that 
of the 10% myotome (sp.v. 6 to sp.v. 14) help to form the »hypo- 
glossus«. 
It is evident that, besides the abducens which has been dis- 
cussed above, there is only one ventral nerve lacking in the Ammo- 
coetes of P. dorsatus. This nerve appears to correspond to s of 
FÜRBRINGERS nomenclature and his /, «, vo, w, ete. are all present. 
The series of dorsal nerves is complete in P. dorsatus. Since accord- 
ing to FÜRBRINGER, Ddellostoma has a larger number of ventral 
oceipital nerves than Petromyzon, it is possible, that in that form 
we shall find the complete and unbroken series of both dorsal and 
ventral nerves present as they were in primitive vertebrates. 
The relations of the dorsal nerves in the branchial region de- 
serve an additional word. We must pieture to ourselves not only the 
sub-branchial muscles but also the skin of the region ventral to the gill 
slits as retaining in a general way the position which it held before 
the great expansion and shifting backward of the gill sacs took place. 
The only part of the skin affeeted by the shifting of the gill sacs 
is that through which the gill openings have actually wandered, namely 
the area ineluded in the groove which connects the gill openings. 
Consequently the skin ventral to the gill openings belongs to the 
proper area of innervation of the ventral rami of the successive 
dorsal nerves. On their way thither, however, they come into con- 
tact with the epibranchial trunk which lies directly in their path. 
Primitively sp.d. 1, 2,3, 4,5 gave eutaneous components to X.br. 2, 
3, 4, 5, 6 respectively, but with the expansion of the gill sacs those 
branchial nerves are spread through a ceutaneous area of ten more 
segments, to sp.d. 15. Now the components in the branchial nerves, 
derived from sp.d. 7—5, are unequal to the innervation of this large 
cutaneous area and one of two things must happen. Either the 
ventral rami of dorsal nerves encountered by the epibranchial trunk 
as it extends backward must go with it around behind the gills to 
innervate the ventral territory, or they must cross over the trunk 
and join themselves to the branchial nerves. Apparently both things 
have happened. Certainly a large or the larger part of the eutaneous 
area below the gill slits is innervated by way of the branchial 
nerves. So far as my evidence goes, nearly all of it is so inner- 
vated. It is possible that at first the eutaneous nerves followed the 
hypoglossus and that after the epibranchial trunk became established 
