Johnston, Morphology of the Head. 263 



nerve root passed behind (caudal to) the myotome of the seg- 

 ment to which it belongs. 



c. Various shiftings of organs have taken place, due to 

 (i) the expansion of the branchial apparatus and (2) the form- 

 ation of a rigid cranium with the consequent loss of dorsal mus- 

 culature. The shifting which may be observed during the 

 ontogeny is a valuable source of information which should be 

 further studied. 



d. Largely on account of the peculiar relations with the 

 ear, the nerve roots connected with the medulla oblongata have 

 shifted backward during the phylogeny, and have left two 

 neuromeres without nerve roots. 



e. As a result of the expansion of the branchial apparatus 

 the visceral sensory and motor components of the nerves caudal 

 to the glossopharyngeus have been collected into a common 

 root, the vagus. The somatic sensory and motor components 

 of more or fewer of these nerves are present as rudimentary or 

 complete dorsal and ventral roots which occupy the posi- 

 tion of the original nerves. The number of these nerves 

 retained depends upon the reduction of myotomes and 

 cutaneous area in the postauditory region. In cyclo- 

 stomes all the general cutaneous roots are present and 

 independent; two of the somatic motor roots have disap- 

 peared. In gnathostomes, owing to the shifting backward of 

 the vagus root at its attachment to the brain, the general cu- 

 taneous root following it has been absorbed and appears in the 

 adult as the second r. auricularis vagi. 



f. There has been no shifting forward of spinal nerves 

 into the oblongata. 



6. The typical sense organs of vertebrates fall into two 

 categories: the neuromast organs constituting the acustico-lateral 

 system, and the end buds or taste buds. These two sets of or- 

 gans are absolutely distinct in structure, function, innervation, 

 and central nerve relations. 



7. It has previously been shown that the acustico-lateral 

 system was derived from the general cutaneous. It is thought 

 that the neuromast organs appeared first in a marginal portion 



