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. r. 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-hyoicl segment is double 

 with respect to its original veno -pericardial muscles as well as in 

 other respects. 



The anterior group of tubular muscles (mt h 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 

 oblicpie 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, i.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 Vllth 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 



