MOTOR NUCLEI IN PHYLOGENY 517 
The entire motor VII nucleus in all sharks lies characteristi- 
cally some distance caudad of the abducens nucleus and the 
rostral end of the motor facial nucleus is placed either caudal 
to or at the level of the last emergent abducens root. On the 
other hand, in all ganoids the rostral end of the motor VII 
nucleus overlaps the abducens nucleus and its emergent root- 
lets for a considerable distance (figs. 17 and 25). It thus be- 
comes apparent that the motor VII nucleus of ganoids lies on a 
more rostral level in the medulla than does this nucleus in 
sharks. In this respect ganoids resemble teleosts (vide infra). 
In the discussion of this region in selachians, the important 
part played in the formation of the ascending motor root of the 
facial and glossopharyngeal nerves by the development in these 
animals of a caudally situated communis nucleus, has already 
been indicated. In Polyodon the communis system seems not 
to be so highly developed as in sharks but to be overshadowed 
in relative importance by the large and extensive acusticum and 
lateral line area (figs. 11 and 13 with 19 and 20). As a some- 
what similar condition obtains in Lepidosteus and Acipenser, 
this may account in part at least for the less caudal position of 
the motor VII nucleus in these forms than in sharks. This ques- 
tion is considered further in the discussion of the motor V nucleus 
(vide infra). 
In this connection it may be noted that the structure of the 
brain stem of Polyodon furnishes no evidence indicative of a 
gustatory function for the ‘primitive pore’ elements which are so 
numerous on the head and bill of this animal (Kistler, 76; Col- 
linge, 13). On the other hand, the highly specialized and 
extensive acusticum and lobus lineae lateralis in Polyodon fur- 
nish strong presumptive evidence in favor of these peripheral 
organs being of the functional nature of neuromasts. As Wright 
(102) originally employed this term to distinguish pear-shaped 
hair cells from rod-shaped taste cells, and since the sense-cells 
of Kistler’s figures strongly resemble the latter elements, the 
term neuromast is here used in a strictly functional sense, i.e., 
to indicate organs centrally related by afferent nerves to the 
special somatic area of the rhombencephalon. 
