520 DAVIDSON BLACK * 
Further, in Acipenser, Polyodon and Amia the expiratory 
branchial muscles are relatively more highly developed as com- 
pared with the expiratory opercular muscles than is the case 
among teleosts. Also, the m. adductor hyomandibularis (ex- 
piratory opercular muscle) in Acipenser, Polyodon, and Amia 
is less highly differentiated than among teleosts (Vetter, 95; 
Danforth, 16; MeMurrich, 83). 
Thus on purely anatomical grounds, it would appear that the 
respiratory cycle in ganoids must present a curious admixture of 
selachian and teJeostean characters. In inspiration these char- 
acters are chiefly teleostean, viz., contraction of the m. sterno- 
hyoideus and the opercular dilator muscle, together with the 
valvular action of the free border of the operculum. In expira- 
tion, on the other hand, selachian characters predominate, viz., 
the action of the relatively well developed branchial levator and 
adductor muscles in conjunction with that of the adductor 
hyomandibularis, and the necessarily coordinate action of the 
m. adductor mandibularis."! 
Turning now to a consideration of the arrangement of the 
motor nucleus in the light of these observations, it becomes evi- 
dent that the caudal extent of the motor V nucleus, together 
with the rostral position of the motor VII nucleus, brings the 
two nuclei innervating the opercular (and jaw) musculature into 
closer relation to one another than is the case among sharks 
where ap operculum is lacking. In this respect these nuclei 
present teleostean characters.” 
On the other hand, the selachian arrangement of the motor 
VITI-IX—X nuclei to form the caudal viscero-motor column ap- 
pears to be directly correlated with the relative importance of 
the intrinsic branchial muscles among ganoids and with the es- 
11 The closure of the mouth during expiration is much more important in 
ganoids than in sharks owing to the teleostean arrangement of the branchial 
lamellae in the former animals. 
Tt is of interest in the present connection to refer to the peculiar arrange- 
ment of the motor nuclei in Chimaera (Kappers, 66, fig. 20). In this animal, in 
which the operculum is also developed, the isolated rostral portion of the motor 
VII nucleus charted by Kappers bears a relation to the caudal motor V ele- 
ments in all essentials identical with that obtaining in ganoids. 
