142 IOWA ACADEMY OF SCIENCE 



second branchial nerve. It forms a small pharyngeal-pretrematic branch, 

 a posttrematic branch of communis fibers only, while the main portion of 

 the nerve after receiving an anastomosis from the second branchial sup- 

 plies the levator and depressor muscles of the third gill. In Spelerpes the 

 nerve is much more rudimentary. It arises as in Necturus close to the sec- 

 ond branchial nerve, but it is so small that its exact distribution was not 

 determined. The levator and depressor muscles of the third gill are 

 supplied by a branch of the second branchial, the one which in Nec- 

 turus anastomoses with the third branchial. The three branchial nerves in 

 Necturus are seen to have essentially the same arrangement and ana- 

 stomoses \ as in Amblj^stoma. Spelerpes differs only to the extent that 

 the third branchial nerve has undergone a greater reduction. The rami 

 laterales X and the ramus intestino-accessorius X have the character- 

 istic arrangement and branching as seen in urodeles in general. 



In respect to the formation of the hypoglossal nerve Necturus and 

 Spelerpes show important differences. In Necturus the two anterior 

 spinal nerves are devoid of ganglia and the hypoglossal nerve is formed 

 by the fusion of ventral branches of these two spinal nerves, with ap- 

 parently a contributory twig from the third spinal nerve. In Spelerpes 

 the second spinal nerve possesses a ganglion, and the hypoglossal nerve 

 is formed chiefly, if not entirely, from the first spinal nerve. 



On considering the cranial nerves of Necturus in their entirety, and 

 comparing them with the corresponding structures in other Urodela 

 that have been studied, we see that their characteristics are not prim- 

 itive but of a rather highly specialized order. Both as larva and adult 

 Necturus resembles in its nerve arrangement the larval stages of Ambly- 

 stoma and particularly of Spelerpes, rather than the more generalized 

 and we may say more primitive type found in the perennibranch Siren 

 and the derotreme Amphiuma. In some respects the Plethodont type 

 of the Salamandrids is the least primitive. Only three branchial arches 

 develop in the larval stage. Necturus in possessing but three branchial 

 arches shows a very distinct resemblance to the Plethodont form. 



There are some strong objections to considering Necturus as a larval 

 Plethodont. Necturus lacks a Jacobson's organ; its r. alveolaris VII 

 does not enter a canal in the lower jaw ; its r. jugularis VII passes out 

 dorsal rather than ventral to the squamoso-columellar ligament; its sec- 

 ond spinal nerve possesses no ganglion. Then, too, the presence of lungs 

 is another objection of great weight. 



But whatever may be the true relationship of Necturus a study of its 

 cranial nerves does not lead one to consider it a primitive urodele. The 

 characters of its cranial nerves are those that are characteristic, with a 



