192 NATURAL SCIENCE. „^,, 



It is on this bony bridge, entirely foreign to the anatomy of an 

 ordinary fish, that the dorsal fin supports itself in its advance 

 towards the snout. Properly speaking, the left side of the face in a 

 plaice, for instance, extends from the ventral edge, or chin, to the line 

 between the eyes ; but the dorsal fin in its anterior extension divides 

 this side of the face into two parts. 



The colour of the skin in a flat-fish is the combined effect of 

 three distinct kinds of tissue. On the inner side of the skin is a dense 

 continuous opaque layer, with a white reflecting surface ; this may 

 be called the argenteum. Near the outer surface of the skin are two 

 sets of microscopic elements — the ividocytes, which are fixed in form, 

 and, like the argenteum, opaque and reflecting, and the chromatophoyes, 

 which contain pigment, and are able to expand and contract their 

 radiating processes. Both argenteum and iridocytes are present on 

 the lower side as on the upper ; it is only the chromatophores which 

 are absent on the lower side. 



The origin of the structural peculiarities in each individual 

 Pleuronectid by a process of metamorphosis is also within the region 

 of verifiable fact. In the majority of genera the lower eye moves 

 apparently round the edge of the head, remaining always at the 

 surface, and always functional. When the transformation takes 

 place in this way the dorsal fin terminates behind the region of the 

 eyes up to the moment when the lower eye has reached the upper 

 side. Then it advances and, as it were, cuts off the retreat of the 

 eye which has (apparently) migrated. In certain cases observed by 

 Steenstrup and Alexander Agassiz the dorsal fin extends between 

 the eyes, while there is still one on each side of the' head. 



The lower eye then seems to pass through the head to reach the 

 upper side. Steenstrup, through insufficient knowledge of the anatomy 

 of the adult flat-fish, concluded from this observed passage of an eye 

 through the head that the lower eye passed under the frontal bone of 

 its own side to reach its final position. But Agassiz, with truer 

 insight, explained that the eye merely passed through the base of the 

 dorsal fin, which in this particular instance (the genus Plagusia) 

 extended forwards before the change in the position of the eyes, 

 instead of after that change. 



The disappearance of the chromatophores from the lower side of 

 the flat-fish also takes place during the individual metamorphosis. 

 The larva has chromatophores on both sides, as it has an eye on each 

 side ; but when the fish has taken to lying on one side on the bottom, 

 the chromatophores on the lower side disappear. 



These primary and essential facts of the peculiarities of the adult 

 flat-fish and their development in the individual are at the present 

 day well-known, and generally accepted. When we pass on to the 

 deeper question of the explanation of these facts, we can likewise 

 formulate certain broad propositions which no modern zoologist will 

 dispute. But the statement that flat-fishes were evolved from 



