MOTOR NUCLEI IN PHYLOGENY 519 
ventricular gray and is placed wholly caudad of the rostral 
border of its emergent root. In its dorsal position this nucleus 
resembles that of sharks, but in its extent caudad of its emergent 
root the nucleus recalls the condition obtaining in cyclostomes 
and, in a manner of speaking, it foreshadows the more com- 
plicated relations which are characteristic of the motor V nucleus 
among teleosts. 
It is an unfortunate fact that the mechanism of branchial 
respiration has not been investigated in ganoids, although the 
air-breathing propensities of these animals have been described 
and recorded by many observers.'° However, a study of the 
arrangement of the musculature of the branchial area among 
ganoids and teleosts makes it evident that, though an operculum 
is characteristically developed in both groups, yet in the rela- 
tive specialization of this apparatus as a respiratory organ, the 
two groups present important differences. 
Among most teleosts the operculum acts in conjunction with 
the maxillary, mandibular and branchiostegal valves as a highly 
efficient pumping mechanism which maintains the flow of water 
from the oral cavity outwards over the gills through the oper- 
cular cleft. In consequence of this, the intrinsic branchial 
musculature among teleosts is relatively poorly developed, while 
the respiratory action of the m. adductor mandibularis (closure 
of the mouth during expiration) is largely abolished (vide infra). 
In ganoids, on the other hand, though rudiments of the maxil- 
lary and mandibular valves may be represented (Allis, 2), yet 
their efficiency as such is negligible, so that during expiration the 
mouth must be closed to direct the respiratory flow backwards 
over the gills. 
10 The only observations on the respiratory movements of the.gills in gan- 
oids that I have been able to find in the literature are those of Kouliabko (77). 
This author carried out a series of experiments upon the isolated heads of various 
fish, among which were specimens of Acipenser ruthenus and another closely 
related form which is referred to as the ‘costeur’ and probably was the great 
Russian sturgeon, Acipenser husio. No exact study of the mechanism of bran- 
chial respiration was made, but Kouliabko brought to light the interesting fact 
that ganoids are more tolerant of CO» than are teleosts. The interest of this 
observation is increased in view of Baglioni’s finding that selachians resemble 
ganoids in this respect (7). 
