EVOLUTION OF ASYMMETRY 



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second group, typified by Pleuronectes, the mouth is much smaller, and the whole 

 jaw apparatus is more strongly developed on the blind side (Fig. 13c). The teeth 

 are much more developed on this side of the jaws, and are frequently entirely 

 wanting on the ocular side. These are the forms which have taken to feeding 

 on the bottom, mainly on molluscs and other invertebrates. The mechanism by 

 which the torsion of the jaws has been brought about has been well described by 

 Cole and Johnstone (1902, p. 30) in Pleuronectes. In the Soleida; and Cymoglossidae 

 the mouth is invariably small, and the lower jaw is never prominent ; the jaws are 

 markedly asymmetrical, those of the blind side being much better developed, and are 



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. 14. — A, Opercular apparatus of Hippoglossus hippoglossus ; B, upper dermal lobe of same 

 (deflected) ; c, branchiostegal membrane of same, showing the sliding valves. [After 

 Schmidt], cl., a smooth projection above the clavicle to which the upper dermal lobe is 

 firmly applied ; p., pectoral fin ; si., the sliding surface of the emarginate urohyal ; sy., 

 syphon; /.t'., tongue valve ; u.rf./. , upper dermal lobe ; y., sliding valves ; f./., pelvic fin. 



Strongly curved, the convexity of the lower jaw fitting the concavity of the upper 

 (Fig. 13D), Small teeth are present in the jaws of the blind side, whereas those of 

 the ocular side are either edentulous or with a few feeble teeth (Cunningham, 1890, 

 p. 41). These Flatfishes feed largely on the bottom, mostly at night, finding their 

 food, which consists of invertebrates and small fishes, by means of the senses of smell 

 and touch. 



In a valuable paper Schmidt (1915) has described a number of interesting adapta- 

 tions connected with the respiratory apparatus of Flatfishes. This work is, unfor- 

 tunately, written in Russian, and, as it lacks even an abstract in any other language. 



