i6 FLATFISHES (HETEROSOMATA) 



opening appears between some of the anterior interncural cartilages, through which 

 opening the eye passes.' In Solea the eye passes over the top of the head fielow the 

 overhanging extremity of the fin as in ScophthaUnus, but in Symphurus (Plagusia), 

 as was hrst observed by Agassiz (1879), the migrating eye has to pass through the 

 tissues of the head, emerging on the other side close to the other eye.^ 



'I'he muscles of the head in the Heterosomata likewise exhibit a greater or lesser 

 tlegree of asymmetry, and the various crests and hollows on the skull for their attach- 

 ment are generally more prominent on the ocular side. The anterior segments of the 

 dorsal lateral muscles are elongated and bent anteriorly, so that they he on the dorsal 

 surface of the head on either side of the anterior extension of the dorsal fin. The 

 eye-muscles, as well as those connected with mastication and respiration, are markedly 

 ;isymmetrical, especially in those forms which spend the whole of their lives on the 

 sea floor. ^ 



In many Flatfishes the jaws and dentition exhibit a marked asymmetry, but this 

 IS of a secondary nature and not related to the original asymmetry produced by the 

 change in the position of the eyes. Psettodes, which probably lies on the bottom and 

 makes short dashes after approaching fishes, has a larger mouth than any other 

 Flatfish, and is the only genus in which the supplemental maxillary bone is developed 

 (Fig. 7). The jaws and dentition are very nearly equally developed on both sides, and 

 are essentially perch-like, the teeth being strong and pointed. In the sinistral Bothidae 

 the mouth is terminal, with the lower jaw prominent, and the jaws and teeth, if not 

 actually symmetrical, are usually well developed on both sides of the head (Fig. 13A). 

 The PleuronectidE, also with terminal mouths and prominent lower jaws, fall into two 

 main divisions. In the first, of w-hich Hippoglossus is a typical example, the mouth 

 IS large and subsymmetrical, and the dentition more or less equally developed on 

 both sides (Fig. 13B). These are all strongly swimming forms which live mainly on 

 small fishes or other agile creatures, and go in active pursuit of their prey. In the 



^ Emery (1883, p. 405, figs.) ; Kyle (1913, pp. 48, 99). 

 - Kyle(i92i,p. 10,'i, figs.). 



^ See Cole and Johnstone (1902, p. 178, fig., pye-niuscles of IHcuiontttci] ; Cunningham (1890, 

 p. 46, general account of muscles of ^olea). 



