EVOLUTION OF ASYMMETRY 19 



strongly developed sliding valves, etc., on the blind side. There can be little doubt 

 that under normal circumstances, when the fish is lying quietly on the bottom, 

 expiration takes place mainly through the upper siphon, that of the blind side being 

 kept closed. 



In the body the asymmetry of the Heterosomata is considerably less marked 

 than in the head, but is to be seen in the musculature, vertebral column and viscera, ^ 

 as well as in the fins and their supports. Practically all the modifications can be 

 shown to be related to the adoption of a demersal habit, and many of them are 

 correlated with the adoption of progression by undulating movements of the body 

 and marginal fins. In Psettodes, and other acti\-ely predacious forms such as 

 Hippoglossns, the asymmetry of the body is, as a rule, less marked than in 

 typically bottom-living forms such as Pleuronectes and Solea. 



The structure of the lateral muscles of the blind side is essentially the same as 

 that of those of the ocular side, but the musculature is nearly always more feebly 

 developed on the former. Some asymmetry is exhibited in the muscles of the 



Fig. 15.— Opercular apparatus of Fsdivcifs ctumei. a, Ventral view of head with the jaws 

 stretched apart ; B, the same with the jaws nearly closed. [After Schmidt. j (^/., dermal 

 fold closing the cavity below the branchiostegal membranes ; ins., muscle band along the 

 edge of the branchiostegal membrane ; r., branchiostegal rays ; nr., lower sliding edge of 

 urohyal. 



abdominal region, and, in a greater degree, by the muscles connected with the paired 

 fins, gill-arches, jaws and so on. The vertebral column of Psettudes is very nearly 

 symmetrical, but in all other Flatfishes the individual vertebrae exhibit a greater or 

 lesser degree of asymmetry, which is less marked in those of the caudal region, and 

 only slightly developed in the last few caudal vertebra and in the hypural elements 

 of the caudal fin,- According to Kyle (1921, p. 86) the general ettect of these asym- 

 metries is to produce " longitudinal curvatures [or torsion] of the vertebral column 

 and vertical lateral flexures ". The same author has pointed out that the asymmetry 

 of the viscera is acquired at a very early stage in the development of the individual.^ 



1 The taxonomic value of the niodihcations in these organs is discussed in the section devoted 

 to classilication (p. 46). 



^ See Cole and Johnstone (1902, p. 40, etc.). 



^ Schmidt (1915, p. 444) has drawn attention to the asymmetrical position of the anal papilla, 

 which in all Flatfishes, including those in which the vent is placed on the blind side, is situated on 

 the ocular side of the median line. The function of this papilla, which is equally developed in 

 l>oth sexes, is not known. 



