20 FLATKISHES (HETEROSOMATA) 



Ajiart from the anterior part of the dorsal fin, which has been already considered, 

 the median fins (dorsal, anal and caudal) show little asymmetry The pectoral fins 

 are subequal in PseKodes and in a number of other genera. In all the more specialised 

 forms, however, that of the blind side is reduced in size, and may even be altogether 

 wanting {e.g. Soleidne and certain genera of Bothida-). In some of the Soleidae and 

 in all the Cynoglossidae both pectoral fins are absent. The difference in the size of 

 the pectoral fins of the two sides is not accompanied by any marked changes in the 

 pectoral arch, which, apart from a certain degree of flexure, usually exhibits compara- 

 tively little asymmetry'. In the more generahsed genera of Bothida-, Pleuronectida; 

 and Soleidae the pelvic fins are nearly symmetrical, and more or less subequal but in 

 some of the more specialised forms that of the ocular side is placed on the median 

 line of the body, and a little in advance of that of the bhnd side. In other genera 

 this asvmmetry is carried still further, the pelvic fin of the ocular side having an 

 elongate base and forming a fringe along the lower edge of the anterior part of the 

 fish, the fin of the blind side remaining small and short-based. In others, again, the 

 bases of both pelvic fins are elongate (Figs. 24, 25). In some members of the Pleuro- 

 nectid subfamily Rhombosoleinac, and in certain of the Soleidae and Cynoglossidae, 

 the pelvic fin of the blind side is absent. 



The scales of Flatfishes, although similar in shape and size on the two sides of the 

 body, generally exhibit differences m spinulation. Thus, in a large number of genera 

 the scales of the ocular side are ctenoid, those of the blind side cycloid, and in others 

 the scales are ctenoid on both sides, but the spinules are nearly always more strongly 

 developed on those of the ocular side. In genera in which the scales are partly or 

 entirely replaced by bony tubercles, these are usually more feebly developed or alto- 

 gether absent on the blind side. In many of the Soleidae, fishes which mostly feed 

 by night, relying almost entirely on the senses of smell and touch to find their prey, 

 the blind side of the anterior part of the head is provided with flexible tactile filaments, 

 which probably represent modified scales. Similar filamentous processes are found on the 

 blind side of the head in certain sole-like genera of Khombosoleina?, which also appear 

 to have nocturnal habits — an interesting case of parallelism (Norman, 1926B, p. 257). 

 The lateral line exhibits some asymmetry, more particularly in the head region. 

 It may be noted that the supra-orbital canal of the blind side, when developed, runs 

 close to the corresponding canal of the ocular side along the interorbital bar. i. e. 

 along the morphological median line of the head. In the region of the body the 

 lateral line is well developed on both sides of the body in Psettodes. as well as in the 

 majority of the genera of the family Pleuronectidae and in certain genera of Bothida?. 

 In the Pleuronectid subfamily Pcecilopsettma;, and in a large number of the genera 

 of Bothinae, the lateral line is either very feebly developed and scarcely apparent to 

 the naked eye on the blind side, or is altogether wanting on that surface. In the 

 Soleidjp and Cynoglossid,"E the lateral line may be developed on both sides of the body 

 or only on the ocular side. In many Cynoglossids more than one lateral line may be 

 present on the body, and there may be a complicated anastomosing system of canals 

 on the head. 



The general absence of pigment on the blind side is another of the characteristic 

 features of the Heterosomata. In certain forms a few spots, blotches or bars of black 

 or brown may be normally present on the blind side, or this may be partially or nearly 

 completely stained with brown or grey, but in the vast majority of Flatfishes, apart 

 from the ambicolorate examples to be mentioned in the next section, this surface is 

 unpigmented. That this asymmetn.' of colour is correlated with a demersal habit is 

 clear, and is confirmed by a study of the development of individual Flatfishes. The 

 pelagic lar\-a. with an e)-e on either side of the head, usually has chromatophores on 

 both sides of the body, but as soon as it takes to lying on the bottom those on the 

 lower surface disappear. In some forms, however, the larva appears to be unpig- 

 mented until It becomes a bottom feeder (Williams, iqo2, p. 10). 



Mention may be made here of the Greenland Halibut (Reiyihardtius), in which the 

 body of the adult fish is generally speaking more plump than in most other Flatfishes, 



