PARKER: OPTIC CHIASMA IN TELEOSTS. 229 



be dorsal. It is thus evident'that the Pleuronectidae, unlike all other fishes, 

 do not have a dimorphic condition of the chiasma, but a monomorphic one, 

 in that destral species, have the left nerve dorsal (Fig. 4) and sinistral 

 species the right nerve dorsal (Fig. 3). This monomorphic condition 

 sets the Pleuronectidae in strong contrast not only with the symmet- 

 rical teleosts, but also with the Soleidae, and justifies the recent tenden- 

 cies in the taxonomy of fishes to separate these two groups. 



So far as the species of Pleuronectidae thus far examined are con- 

 cerned the generalization reached in the preceding paragraph may be 

 put in a still simpler way. In the sinistral species the right eye is the 

 one that migrates and its nerve, as we have seen, is always dorsal ; in 

 the dextral species the left eye migrates and its nerve is likewise dorsal. 

 Hence in all Pleuronectidae thus far considered the nerve of the mi- 

 grating eye is dorsal. This conclusion was reached by Williams (:02, 

 p. 34) for the two species studied by him, and, as the preceding account 

 shows, it probably applies generally to such species of the Pleuro- 

 nectidae as are exclusively dextral or sinistral. 



There is a certain mechanical advantage in the dorsal position of the 

 nerve of the migrating eye. Since this eye moves through the dorsal ' 

 part of the head, its nerve is in a more advantageous position to move 

 with the eye if dorsal at the chiasma than if ventral. With the 

 nerve dorsal the effect of the migration, as already pointed out, would 

 be to bring the two optic nerves into more nearly parallel positions, that 

 is, to make the chiasma less emphasized than in a symmetrical fish, as 

 Cole and Johnstone (:01, p. 117) have already observed it to be m 

 Pleuronectes platessa. Were the nerve ventral, the effect of the migra- 

 tion would be to wrap it around its fellow so as to accentuate the chiasma. 

 While this latter condition is not impossible, for, as we have seen, it 

 exists in many of the Soleidae, it is certainly less advantageous mechani- 

 cally than the other. One may, therefore, say that the monomorphic 

 condition of tlie Pleuronectidae is of such a kind as to give a mechanical 

 advantage to the migrating eye. 



The crossing of the optic nerves in young Pleuronectidae is established 

 in the eggs long before the young fishes hatch and is, I believe, as 

 uniformly monomorphic there as in the adults. It is well known to all 

 who have had any experience in rearing young flounders that their 

 period of greatest mortality is during the migration of the eyes. It 

 might be supposed that those which die at this stage are flounders whose 

 migrating eyes had ventral nerves; that, in other words, the flounders 

 hatched from eggs included animals with the nerve of the migrating eye 



