569 



The next gill behind the mouth has also been suppressed, but the 

 spiracular cartilages bear evidence of its original existence. The fol- 

 lowing cleft is the hyoid, and its arch has been preserved, the hyoid 

 with the hyo-mandibular. There is another hiatus below the auditory 

 segment, but thereafter there is an uninterrupted series of branchial 

 clefts and arches, which however phylogenetically has suffered re- 

 gression from the posterior end. 



This is borne out, not merely by a consideration of the original 

 segmental arrangement of the nerves, but by the condition of the epi- 

 branchial gangUa of the Ammocoetes (v. Kupffer etc.). v. Kupffer 

 found four epibranchial ganglia in the region of the trigeminal, and 

 drew attention, moreover, to the epibranchial-like origin of the lens in 

 front of the series. The three posterior of these were evidently as- 

 sociated with gill clefts, all involved in the Craniate mouth. As a 

 rule it is only the last of the series which develops. These three 

 then may be apportioned to the three primary ganglia and nerves of 

 the trigeminal. The only nerve which occupies the position demanded 

 by the anterior fourth epibranchial ganglion is the trochlearis. 

 V. Kupffer found this nerve associated with the anterior ganglion of 

 the trigeminal group, just above the epibranchial ganglion in question. 

 In other forms the trochlearis has been noted by many writers to be 

 developed in a similar manner to the other visceral nerves. It may 

 after all be the fact then that the IVth and this ganglion were as- 

 sociated primarily with the second oral cleft. If it should be found 

 afterwards that v. Kupffer was not wrong in supposing that the lens 

 might be developed from an epibranchial ganglion, the nerve to which 

 it was related will probably be found to be the mesencephalic still 

 represented in Ammocoetes. 



Ammocoetes presents two epibranchial ganglia in front of the ear 

 and behind the trigeminal in association with the facialis. One sur- 

 vives as the epibranchial ganglion of the spiracle. The anterior one, 

 which is suppressed, must have been associated with a gill cleft be- 

 tween the mouth and the spiracle. There is no epibranchial ganglion 

 in connexion with the auditory nerve. Behind that segment there is 

 •a regular succession of seven epibranchial ganglia, the first belonging 

 to the IXth, the rest to the Xth. The last is coincident with the 

 13th rhombomere as described in this and preceding papers. 



It follows from the foregoing that the Craniate position of the 

 ophthalmic V is a new one consequent upon the mesencephalic flexure. 

 It is primarily related to an oral gill cleft and posterior to the optic 

 segment. It is also suggested that the ophthalmic VII is a nerve 



