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Part III. — Eighteenth Annual Report 



Nasal Organ. 



The nasal organs show important "differences in the various groups 

 under consideration, even superficially. If the animals are regarded 

 from in front, it will be at once remarked that the position of the nasal 

 organ of the blind side relative to the anterior extremity of the dorsal 

 fin is not the same in all the groups. These differences in position are 

 related to the varying amounts of torsion which the organ of the blind 

 side has undergone during the migration of the eye of that side. 



In most species of the Halibut- and Citharus -groups the nasal organ 

 of the blind side lies on the ridge of the head, i.e. on the median 

 longitudinal line of the body, and directly in front of the anterior 

 extremity of the dorsal fin (Plate XI., Fig. 2, l.n.o. and D). In these 

 forms both sides of the mouth are equally and almost symmetrically 

 developed, and the torsion of the head caused by the migration of the 

 upper eye from the blind side has thus had the effect of turning the 

 nasal organ through a right angle independently of the jaws, which have 

 retained their original positions. The dorsal fin does not come so far 

 forward in these as in the other groups, so that the position of the 

 nasal organ has been determined almost solely by the migration of 

 the upper eye. This conclusion is corroborated by what we find in the 

 Lesser Halibut {Platysomatichthys hippoglossoides). In this, the jaws 

 are as in the Halibut, but the left eye does not complete its wandering 

 to the upper side. It rests on the ridge of the head, and associated 

 with this the corresponding or left nasal organ remains on the blind 

 side, although close to the ridge. 



In the Piaice-group, the mouth is small, and the lower side is much 

 more developed than the upper. There is thus a torsion of the mouth 

 downward, and hence an apparently greater relative torsion of the left eye 

 and, with it, of the left nasal organ upward, from the blind side towards 

 the right or upper side. In reality, however, the nasal organ of the blind 

 side is in almost exactly the same position as in the groups previously 

 mentioned, I.e. it is on the median dorsal longitudinal line of the body 

 (Plate XI., Fig. 1, l.n.o.). But the dorsal fin grows forward and inclines 

 towards the blind side of the head as it approaches and passes alongside 

 the eye of that side. The nasal organ of the blind side may therefore 

 be said to be on the upper or eyed side of the dorsal fin (Fig. 1, D, and 

 l.n.o.). In some cases, e.g. P. cynoglossus, the nasal organ in question 

 lies obliquely on the ridge anterior to the dorsal fin. 



In the Turbot-group the mouth is symmetrical as in the Halibut- and 

 Citharus-groups. There is consequently no torsional influence from 

 this side on the position of the upper or right nasal organ. Hence it is 

 curious to find that, though the torsion of the eye seems to be more 

 complete here than in the other groups, yet the nasal organ is not 

 twisted round so far. This difference has arisen through the great 

 development anteriorly of the dorsal fin. The early stages in develop- 

 ment of several species of this group are fairly well known, and it has 

 been repeatedly shown (19, 26, 45) that the dorsal fin grows forward 

 ere the emigrating eye lias had time to pass over the ridge of the head. 

 In some cases the anterior extremity of the fin overhangs the eye as it 

 passes the ridge ; in other cases the eye seems to pass through the 

 tissues of the base of the fin which has grown so far forward. It is not 

 astonishing, therefore, that the dorsal fin should prevent the complete 

 torsion of the nasal organ of the blind side, although it was not suc- 

 cessful in barring the way to the eye. Hence we find that the nasal 

 organ is on the blind side of the dorsal fin (Fig. 3, r.n.o. and D). 



In the Sole-group we find a condition of things which almost seems 



