CRANIAL NERVES OF SIREN LACERTINA 335 
that the ramus is exclusively sensory and primarily general 
cutaneous. 
In comparing the branchial region in Siren with that of other 
Urodela we must consider the larval stages of the latter. We 
then see that in Siren, as stated in foregoing sections, there has 
occurred a considerable modification in the posterior branchial 
region.. The fourth and fifth branchial nerves have largely 
disappeared, and to a less degree the third, and to a great extent 
have been replaced functionally by the ramus intestinalis re- 
currens X, which may be interpreted as the much hypertrophied 
ventral portion of still more posterior branchial nerves. 
We may hope to appreciate the real significance of the seeming 
peculiarities in nervous and other structures in Siren only as we 
work out more exactly their comparative anatomy. Before 
generalizations can be made safely there must be carried on in- 
vestigations of the cranial nerves and associated structures of 
other Urodela, such as Cryptobranchus, Necturus, Salamandra, 
etc. Studies upon these and upon the Caecilia will doubtless 
throw considerable light upon the problems connected with the 
peripheral distribution of the cranial nerves of the Urodela. A 
study of the cranial nerves of Siren confirms the conclusion 
drawn from a consideration of its general structure that it, like 
Necturus, is a specialized rather than a primitive form, but that, 
nevertheless, there may be recognized in it structures and charac- 
ters that suggest persistences and survivals from an ancestral 
condition. Drier would derive the Urodela from a pre-selachian 
stock; Bender sees in the selachians, through the crossopterygians 
and dipnoans, the urodelan phylogeny. Obviously a study of 
Siren can aid but little in such theoretical considerations. 
