| 326 H. W. NORRIS 
ability in its so occurring in some instances. For some distance 
its course is along the medial border of the fourth branchial arch 
to which it contributes pretrematic branches. 
10. The fourth and fifth branchial nerves 
On the view that Siren is not a primitive form, but a perma- 
nent larva, we see an explanation of the fact that in this species, 
as in the larval stages of caducibranchiate Urodela in general, 
great reduction has occurred in the posterior branchial nerves. 
A typical branchial nerve of the Urodela, as outlined by Wieders- 
-heim (’77) and by Driiner (’03), may be described in the terms 
of the nerve-component theory as follows: on leaving the ganglion 
the nerve trunk divides into (1) a ramus posttrematicus of motor, 
communis and general cutaneous fibers, which runs, posterior 
to its gill slit, along the lateral border of its corresponding branch- 
ial arch; (2) a ramus pretrematicus of communis fibers only, which 
runs, anterior to its gill-slit, along the median border of the next 
anterior branchial arch; (3) a ramus pharyngeus, at first united 
with the ramus pretrematicus, of communis fibers only, which is 
distributed to the dorsal pharyngeal wall. In the larval stages 
of most of the existing Urodela are found four well developed 
branchial arches, but the corresponding branchial nerves are not 
equally well developed. The first (1X), second (X./) and third 
(X.2), as in Siren, show the characteristic rami, but in the third 
there is commencing a reduction in the ventral motor constituent — 
ramus. In the other, more posterior, branchial nerves there is a 
complete loss of the ventral portions of the ramus posttrematicus, 
or of the entire ramus, or a great transformation which obscures 
the original condition. As noted by Driiner (’04, p. 425), the 
nerves are the most conservative structures of the branchial 
region. They will therefore constitute more reliable guides in 
the search for the primitive relations in this region than will the 
branchial arches themselves. 
Driiner finds in Siredon a fourth branchial nerve with a dis- 
tinct posttrematic ramus. From this nerve a branch is given off 
which bears such a relation to the rudiment of a fifth gill-slit 
