4 



eyes of the consumer. Again, more than one kind of sole is sometimes sold under 

 that name. 



It is evident, then, that before we commence the study of the common sole we must 

 make ourselves accurately acquainted with its special features, so that we may not 

 mistake other kinds for it, and may identify it with certainty at all stages of its 

 existence. In order to discover these special features we must compare all the kinds 

 of flat-fishes with other fishes and with one another and thus ascertain — 1st, what 

 features are common to all of them ; 2nd, what features are common to certain divi- 

 sions of them and especially to the division which includes the sole ; 3rd, what are the 

 differences between the several kinds or species included in this particular division. 



In order to study the characters of the flat-fishes we must of course give names to 

 their various organs ; but all the bony fishes are made up of the same organs in 

 different shapes and sizes and in various proportions to one another. Thus we have 

 only to examine the sole and compare it with a fish of the more usual structure in 

 order to recognise the various organs which have long borne appropriate names. 



In any fish of the ordinary type, for example, a salmon, lierring, cod, perch, 

 mackerel, &c., the two sides resemble one another in form, structure, and colour, and 

 correspond to one another in the position and direction of their component organs. 

 In other words, all the organs, with the exception of some of the internal, are in pairs, 

 the two members of each pair being situated on opposite sides of the middle plane of 

 the body, at the same level, and at equal distances from it. This plan of structure 

 occurs in the majority of fishes and nearly all the otlier vertebrates, and also in the 

 greater number of the lower animals, for instance in Crustacea and insects. To 

 obtain a distinct idea of it we may reflect that the body of any animal in which it 

 occurs, say a normal fish, a dog, or a man, has length, thickness, and breadth. The 

 length is measured from tlie head to the posterior end of the body, the thickness from 

 the back to the ventral surface, tlie breadth from side to side. Now the organs at 

 opposite ends of the line which measures length are utterly different in form and 

 structure, and those at the back are equally diflerent from those opposite to them at 

 the ventral surface. But, wherever we measure the breadth, the parts at one end of 

 the line along which we measure will be exactly similar in form and structure to 

 those at the other, but exactly reversed in horizontal direction, This structural 

 feature is called bilateral symniistry. 



The greater number of fishes are bilaterally sj-mmetrical, and the line along the 

 surface which separates the two similar halves passes between the eyes at an equal 

 distance from each. If we look for the line which divides two similar halves in the 

 sole we shall not be able to find it. It is obvious that a line drawn between the eyes 

 does not divide the head into two equal and similar parts ; but if we look for the 

 paired organs of the ordinary fish in tlie sole we shall find in most cases that one of 

 each pair is on the coloured or upper side of the sole, and the other on the white or 

 lower side, but that in many cases the two members of each pair are not so exactly 



