CHAPTER X 

 FLAT -FISHES— Continued 



In this chapter the other groups of the flat-fishes (Heretosomata) 

 will be considered in relation to their feeding habits and their brain 

 pattern ; it will be found that these groups conform to the recent 

 classification of Norman and are supported by the work of Ford on 

 the vertebral column. 



The second of our groups consists of certain species of the family 

 Pleuronectidse and are as follows : 



The plaice, Pleuronectes platessa ; the flounder, P. flesus ; 

 the sand dab, Hippoglossoides ; the lemon sole, P. microcephalus ; 

 the witch, P. cynoglossus. 



Plate 13, Figs. 2-5 contains outline drawings of the naked-eye 

 appearance of the brain of lemon sole, plaice, witch, and dab. The 

 characters of this group are best exemplified by the plaice. When 

 the drawing of the plaice is compared with that of the sole, the 

 most striking difference to be noted is the large size of optic lobes 

 each of which is bi-lobed in an antero-posterior plane. The primi- 

 tive end-brain is, on the other hand, much smaller. The somatic- 

 sensory lobes, instead of having an anterior bulge, and appearing 

 like two semi-colons facing each other, as in the sole, are narrow 

 and elongated, and separated by a deep cleft. 



It is now necessary to see what is the evidence relating to the 

 acoustic and gustatory functions in the plaice. This can be fur- 

 nished by an examination of serial sections ; but before doing so 

 the sections of the optic lobes should be examined and compared 

 with similar sections of the sole. 



The two large scale drawings of sections across the middle of 

 the optic lobes show the very great prominence of the tectum 

 opticum of the plaice compared with that of the sole ; these drawings 

 .also show the details of the lobi inferiores to which we shall refer later. 



In the plaice the facial nerves are of considerable size and 

 pass transversely inwards before turning caudally. The facial 

 lobes are large, and form a slight projection into the walls of 

 the ventricle ; they are capped by the fifth lobes and do not 

 appear in a naked-eye view ; their descending fibres form a strong 



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