similar as they are in the ordinary fish. Thus in the sole there is a giU-cover or 

 operculum on the upper side and another on the lower, and beneath each is a similar 

 gill-apparatus ; half the mouth is on the upper side and half on the lower ; there are 

 two nostrils on the upper side and two on the lower ; a pectoral fin and a pelvic fin on 

 the upper side and two corresponding fins on the lower ; scales on the upper side and 

 similar scales with a corresponding arrangement on the lower ; and finally a line of 

 peculiar scales, a lateral line, along the middle of the upper side and a similar line on 

 the lower. It follows, therefore, that the long continuous fins along the two edges of 

 the sole correspond to the median fins of the ordinary fish, and as we find the anus and 

 the junction of the gill arches on one edge of the sole, this edge corresponds to the 

 ventral median line of the ordinary fish, so that the elongated fringing fins of the 

 sole are the dorsal and ventral median fins respectively ; the ventral median fin is 

 generally called the anal fin. We thus find that the upper side of the sole is the right 

 side and the lower the left, and the edge on which the anus opens is the ventral edge, 

 the other the dorsal. But the two eyes, though in other respects similar to the two 

 eyes of an ordinary fish, are both on the right side, one nearer to the dorsal edge, the 

 other nearer to the ventral. We may distinguish these eyes as the dorsal and ventral 

 respectively, and even without further knowledge we might consider it probable that 

 the dorsal eye corresponds to the left eye of an ordinary fish and the ventral to the 

 right, and the idea would naturally occur to us that the left eye in the sole had been 

 somehow drawn out of its original position on the left side and carried round to the 

 right side. 



The fins of the sole are all supported by flexible or soft rays ; none of the rays are 

 rigid pointed spines ; but this is a character which occurs in certain symmetrical 

 fishes, for example, the cod and whiting. The possession of a single elongated 

 median dorsal fin and a single similar ventral fin is also not peculiar to the sole : some 

 members of the cod family have but a single dorsal and ventral fin. But no 

 symmetrical fish has a dorsal fin extending so far forwards as the sole : in the latter 

 this fin is continued to a point on the edge of the body anterior to the eyes, and in 

 every flat-fish, except one species, it extends at least as far as a poirit above the dorsal 

 eye, while in all symmetrical fishes the limit is somewhat behind the eyes. 



The lower or left side of the sole differs from the right side not only in being white 

 instead of coloured, but also in being flatter, and the fin- rays of the median fins are 

 also more prominent on this side. 



The examination of other flat fishes will show that they resemble the sole and differ 

 from symmetrical fishes in the features just mentioned, if we except the fact that in 

 some kinds it is the left side and not the right which is coloured and which bears the 

 ej'es. Thus the turbot and brill have the eyes on the left side, while the plaice 

 flounder, dab, and halibut, like the sole, have them on the right side. 



All the various kinds of flat fishes thus resemble one another and differ from 

 symmetrical fishes in the particulars now described, which may be thus summarised : — ■ 



