EVOLUTION OF ASYMMETRY 



15 



towards the blind side, and in some specialised genera {e. g. Pleuronichthys) one or 

 more rays of the anterior part of the fin may be on the blind side of the head (Fig. 11). 

 The anterior extension of the dorsal fin in Flatfishes appears to have been effected in 

 one or other of two ways. In some forms the first few interneural spines (interspinous 

 bones) are inclined forwards, so that the first of these may actually occupy a horizontal 

 position, thus carrying the rays of the dorsal fin to the required place ; in others the 

 anterior interneural spines seem to have themselves moved forward along the upper 

 surface of the cranium from a position in the region of the supra-occipital to one on 

 the frontal of the blind side (Fig. 12). In the genus Solea the anterior interneural 

 spines, which are inclined forward, are supported by a special curved spine-like bone 

 lying nearly parallel to the axis of the cranium. This may represent a detached and 

 much modified interneural spine. 



The structure of the anterior part of the dorsal fin suggests that phylogenetically 

 the forward extension in the Heterosomata must have taken place after the two eyes 

 had established themselves on the same side of the head, or, at any rate, after this 



;. 12. — Skeleton of anterior part of dorsal fin and hinder part of skull of a, Pleiironectes 

 platessa ; b, Solea solea. b.d.f., basal bone of dorsal fin ; f.r.d., ray of dorsal fin ; in. sp., 

 interneural spine, (x i.) 



process had been initiated, a vie%v which is fully confirmed by a study of its ontogeny. 

 In most genera the anterior extremity of the fin in the larva lies behind the eyes, in 

 much the same position as that of the first dorsal spine in the adult Psettodes. As 

 soon as the migrating eye has passed over the top of the head the dorsal fin moves 

 forward to take up its final position. In other genera, however, the forward move- 

 ment of the fin takes place before the eye has had time to pass over the edge of the 

 head. In Scophthalmiis and related forms the anterior extremity of the fin may over- 

 hang the eye as it passes over the edge of the head, or the eye may actually pass 

 through the tissues of the basal part of the fin. In Arnoglossiis the anterior part of 

 the post-larval dorsal fin becomes detached from the ethmoid region of the cranium, 

 and the eye passes through the slit thus formed.^ This passage of the eye through a 

 slit between the fin and the head was observed in a living, but unfortunately uniden- 

 tified larval form taken in Japan by Nishikawa (i8gi). In Bothus, another highly 

 specialised genus, the dorsal fin again extends forward before the migration of the 

 eye is complete, but does not become detached from the cranium. Instead, an 



' Kyle (1013. P- 47). See also Facciola t'900, Boll. Soc. Zool. Ital., (2) i, (ix), pp. 169-189, 

 ■ pi.]. 



