M 



1 I.ATFISHES (HETEROSOMATAi 



size of the olfactory lobe and nerve of the ocular side, tlie lirani of tlie adult Flatfish 

 shows no marked asymmetry, and is essentially similar to that of an ordinary Percoid 

 fish. 'I'he anterior part of the brain is twisted to the ocular side, so that both olfactory 

 lobes and nerves lie on the same side of the morphological median line. 



The position of the nasal organs of the two sides of the head exhibits considerable 

 asymmetry in many Flatfishes, and provides a character of taxonomic importance 

 (see p. 37). In Pselloiies the nasal organs are nearly symmetrical, that of the blind 

 side tieing a little nearer to the median line than that of the ocular side. In the 

 remainder of the group the organs are nearly or quite symmetrical in position in the 

 " Soles " (Soleida' and Cynoglossida?), whereas, in the majority of the genera of 

 BothidiC and Pleuronectida?, that of the blind side has accompanied or followed the 

 eye in its migration and lies close to the median line of the head (Fig. 11). As far 

 as the modification of the anterior part of the skull is concerned, such genera as Solea 

 and Cynnglosstis are equally, if not more specialised than, say, Pleuronectes and 

 Scophthahnus. in which the nasal organs are markedly asymmetrical. It is of interest 



I views of A, Psdtndci crumci ; B. Hlppogloss 

 D, Pleuronectes platessa ; E, Scophthahnus maximus ; f, Solea solea ; 

 the relative positions of the nasal organs and the anterior extension of the dorsa 

 dorsal fin ; /., left nasal organ ; r., right nasal organ. 



to note that the only " Flounders " with symmetrical nasal organs are to be found in 

 the Pleuronectid subfamily Khombosoleina;, and that these all agree in having compara- 

 tively small eyes — a character which they share with the "Soles". It seems possible 

 that the symmetrical position of the nasal organs in these fishes may be correlated 

 with the size of the eyes, since the migration of a small eye would occasion much less 

 disturbance of the anterior part of the head than would that of a larger eye.' 



The anterior extension of the dorsal fin on to the head may be conveniently 

 considered here, since its relation to the eyes and nostrils is a character of considerable 

 taxonomic value, and one of some importance to the understanding of the asymmetry 

 of the head. In the generalised Psettodes the first spine of the dorsal fin is situated 

 on the nape and well behind the eyes, but in all other Heterosomata the fin has 

 extended forward at least to above the eye, and in some genera it reaches the extremity 

 of the snout. This extension is along the ridge of the supra-occipital, and thence 

 along the bar formed by the union of the prajfrontal and frontal of the blind side 

 above the upper eye, 1 c. along the pseudomesial bar instead of along the morpho- 

 logical median line. Above the eye the fin is not infrc<]ucutly bent over a little 



an (i<)26b, pp. 



-8). 



