730 ALLTS: [VoL. XII. 
backward, so as to form a pocket, the hind end of which lies 
considerably posterior both to the outer and to the inner edges 
of the septa. The septa and segments in this part run inward 
and backward from the outer surface of the body to the hind 
end of the pocket, and then inward and forward to the mid- 
vertical plane of the body. The cause, whatever it may be, 
that has led to the formation of these pockets is certainly the 
one that has also caused the shifting of the inner edges of the 
septa. A good and sufficient reason would seem to be found 
in the fact that the tail is capable of, and subject to, violent 
movements in early larval stages when the egg-yolk has not yet 
been absorbed. The trunk, or at least its anterior portion, is 
held by the yolk practically immovable, and as something must 
yield when the muscles contract, it is apparently the trunk 
septa, which are thus pulled backward by the functionally more 
active muscles of the tail. If the tail movements began before 
the dorsal arches had become fixed in position, those arches 
would doubtless yield and shift with their septa, the amount of 
the shifting depending upon the state of development of the 
vertebral column when the tail movements first began. The 
varying and perplexing relations of the dorsal arches to the 
vertebral bodies might thus be simply and naturally accounted 
for. 
At its anterior end the limited muscle mass here considered 
extends into the temporal groove and above and below the 
vagus foramen, the vagus and nervus lineae lateralis, as they 
issue, lying in a notch in the front edge of the mass. By this 
notch the first muscle segment is always cut into two portions, 
and the second segment almost but not entirely so. The first 
intermuscular septum, the septum between the first and second 
segments, is not, however, usually cut through; it is simply 
interrupted and pressed back by the issuing nerves toward or 
against the second septum, so that the second segment is 
pinched off at that point. Amia thus shows, in this, practically 
the same arrangement as that described by Hoffmann (No. 57) 
in Acanthias. It seems to me, however, that the vagus cuts 
into the muscle segments, rather than that the latter push for- 
ward above and below the nerve. 

