2>S n.ATFISHF.S (HinEROSOMATAi 



of .1 particular species are either riKht-sided (dextral) or lelt-sided (sinistral), but in 

 a few species reversed individuals occur in var\-ing numbers In the Plaice (Pleiiro- 

 tieclfs) and the Halibut {Hippoglossus), for example, the lish is normally dextral, but 

 very occasionally a sinistral individual is recorded. In the European Flounder 

 iPlatichthvs JIfiiis) and in the Starry Flounder {P. stellatus) of the North Pacific, also 

 normalK" dextral species, reversed individuals are more common. UuncUer, who 

 examined samples of Flounders from various localities, found that the percentage of 

 sinistral individuals varied from 3 to 30. In the Pacific species this percentage may 

 be even higher.' In the so-called Bastard Halibut (Paralichthys californiciis) of 

 California, a sinistral species, individuals with eyes on the right side are almost as 

 common as those with eyes on the left side. Some species of Hippoglossuia and 

 Xvslreurys, also American .genera, are likewise indifferently sinistral or dextral. 



The work of Parker (11)03) on the optic nerves of the Heterosomata, afterwards 

 confirmed by Mayhoff (1912), is of particular mterest, not only in connection with 

 these reversed individuals, but also with the general classification of the group. 'I'hc 

 results of this work may be briefly summarised. In ordinary Bony Fishes the optic 

 chiasma is dimorphic in character, the right nerve crossing above the left about as 

 often as the left above the right.'- In the families Soleidae and Cynoglossida?, and in 

 Piettodes, the chiasma is again dimorphic, and it follows that in these F'latfishes the 

 optic nerves must be partly uncrossed when the nerve of the migrating eye is dorsal, 

 and that they almo.st cross each other twice when this is ventral. In all other Flat- 

 fishes, whether dextral or sinistral, the nerve of the migrating eye is dorsal ; that is to 

 say, in dextral forms (PleuronectidEe) the nerve of the left eye is dorsal, in sinistral 

 forms (BothidcE) that of the right eye is dorsal. The only exceptions to this rule are 

 provided by the reversed individuals just mentioned. In the case of a left-sided 

 individual of a normally dextral genus {e.g. Platichihys), instead of the nerve of the 

 migrating eye being dorsal, that nerve is dorsal which is normally dorsal in the genus, 

 in this case the left nerve. Similarly, although individual examples of certain species 

 of Paralichthys are indifferently sinistral or dextral, as this is normally a left-sided 

 genus, the nerve of the right eye is always dorsal (Fig. 19). 



This monomorphism of the optic chiasma, which is clearly a specialisation, would 

 appear to have been adopted on account of its mechanical advantage (Parker, 1903, 

 pp. Hi), 235). This view is confirmed by the fact that Duncker (1900) has shown 

 that in the Flounder there is a differential death-rate, that of sinistral (i.e. reversed) 

 individuals being higher than that of normal ones. .\s far as the optic nerves are 

 concerned, a reversed Flatfish is not a mirror image of a normal one, and. according 

 to Parker, the monomorphism of the chiasma is established even before the larva is 

 hatched. This fact lends additional support to the view already expressed (]i. 12) 

 that the Flatfish larva is never really symmetrical. 



It is difficult to understand why reversed individuals should be coniparati\ely 

 common in some species and very rare or quite unknown in others. In any case, in 

 view of Parker's conclusions, it is clear that Thilo (1902B) was incorrect in regarding 

 the occurrence of dextral and sinistral individuals in nearly equal numbers as a primi- 

 tive feature. In Psellodes, on the other hand, in which the chiasma is dimorphic as in 

 the " Soles " and normal fishes, the occurrence of dextral and sinistral individuals in 

 nearly etjual numbers is very probably a primitive feature (Regan, 1910B, p. 4."<.S)- 



As already remarked, the reversed Flatfish is not a mirror image of a normal 

 individual of the same species, and it is of interest to find that in the genus Rhombo- 

 solea of Australia and New Zealand there is a tendency to reproduce partially or 

 completely the pelvic fin of the blind side (normally absent in this genus) in reversed 

 examples (Norman, 1926B, p. 280). The single pelvic fin is normally elongate, median 



' I'ruf. Hul.bs h.is poinn-ti out lo inc ui .1 k-ltcr thai 111 Japan he found IHatuhDns iti-llaliis 

 nivanably revtrsed (1. ,-. sinistr.ill, whereas m California tht number of dextral and sinistral 

 individuals was about c<|ual. and in material from Alaska about 75°,, of the hshes were reversed, 

 the maten.d in the British .Museum, although small in numbers, bears out this conrlusion. 

 = In I x.UTipl.-s c-x.uiuned the rnjlit rier\e was dorsal in sij, lb.- I.^ft ni ,|Sl,, 



