MOTOR NUCLEI IN PHYLOGENY Ros 
placing the motor nuclei of the nerves to the hyobranchial 
musculature in intimate association with the chief afferent center 
acting upon them reflexly, in conformity with the first concept 
of neurobiotaxis. 
The most striking characteristic of the motor nuclear pattern 
of elasmobranchs is its relative fixity in all members of the 
group. Thus, as regards their nuclear pattern, the most primi- 
tive shark resembles the most specialized ray much more closely 
than such allied teleostean forms as Tinea and Gadus resemble 
one another. 
However, that this reflex organization within the elasmobranch 
brain stem has been entirely adequate for the needs of these 
animals is indicated by the successful manner in which the 
members of this group have competed with other more specialized 
and modern types in practically every variety of marine 
environment. 
The reduction of the visual apparatus among ganoids has 
apparently been followed by no marked compensatory develop- 
ment of other of the special senses and has been accompanied 
indeed by a great reduction of the functional development of 
the cerebellum as compared with sharks. Apart, however, from 
the relatively small tectum opticum and the reduced cerebellum 
the general organization of the afferent functional divisions of 
the ganoid brain stem is on the whole more selachian than 
teleostean in character. 
The selachian arrangement of the communis area among 
ganoids has necessarily affected the distribution of the elements 
of the caudal viscero-motor column. Thus, except for the gen- 
erally more rostral position of the motor facial elements, the 
viscero-motor nuclear pattern of these animals is essentially 
similar to that of sharks. 
On the other hand, the effector organs of the head region in 
ganoids are modified in general away from the selachian type, 
so that especially in the arrangement of the gill laminae and 
operculum these forms present important teleostean resemblances. 
The peculiar combination of central selachian and peripheral 
telesotean characters noted above is apparently an evidence of 
