300 THE ORIGIN OF VERTEBRATES 
hyoid tubular muscles in the velum gives evidence that the oper- 
cular segment takes part in the formation of the septum, as already 
suggested, 
Miss Alcock, in her paper, speaks of tubular muscles belonging 
to the hyoid segment, which are attached to the muco-cartilage. 
Schaffer also speaks of certain tubular muscles belonging to the velar 
group as piercing the muco-cartilage (h. 7. s.) in his figures 24 and 25, 
i.e. the metastomal bar, near its junction with the opercular bar. In 
my specimens there is a distinct group of tubular muscles which 
pierce the opercular bar of muco-cartilage at its junction with the 
metastomal bar, and pass into the posterior group of velar muscles, 
They clearly belong to the hyoid segment, as Miss Alcock supposed, 
but are not attached to the muco-cartilage. It is possible that they 
represent a different group to those already considered, and suggest 
the possibility that this opercular or thyro-hyoid segment is double 
with respect to its original veno-pericardial muscles as well as in 
other respects. 
The anterior group of tubular muscles (mt, Figs. 116, 117) 
belonging to the same segment as the metastomal bar must now be 
taken into consideration. Very different is their origin to that of 
the posterior group: they arise close up against the eye, and have 
given rise to Kupffer’s and Hatschek’s misconception that the superior 
oblique muscle of the eye arises from a part of the velar muscu- 
lature. Naturally, as Neal has pointed out, they have nothing to do 
with the eye-muscles ; the superior oblique muscle is plainly in its 
true place entirely apart from these velar muscles, which form the 
foremost group of the segmental tubular muscles. They pass into 
the anterior part of the velar folds and run round to the ventral 
side just in the same way as does the posterior group. This anterior 
group of tubular muscles represents the veno-pericardial muscle of 
the segment immediately in front of the opercular, 7.e. the metasto- 
mal segment, and is the foremost of these veno-pericardial muscles. 
Its presence shows that the velar folds, formed as they were by the 
breaking down of the septum, are in reality part of two segments, 
viz. the opercular and the metastomal, which have fused together 
in their basal parts, and by such fusion have caused the inter-relation- 
ship between the VIIth and Vth nerves, so apparent in the anatomy 
of the vertebrate cranial nerves. 
A further piece of evidence that this anterior portion of the velum 
