VERTEBRATA: FISHES. 487 



Turbot, Halibut, Brill, and others, in all of which there is a 

 very curious modification in the form of the body. The body, 

 namely, in all the Flat-fishes (fig. 263) is very much compressed 



Fig. 263. Pleuronectidse. Rhombus fanctatus. Natural size (after Gosse). 



from side to side, and is bordered by long dorsal and anal fins. 

 When young, the body is symmetrical, the eyes are bilaterally 

 situated, and the animal swims in a vertical position. Soon, 

 the habit of lying on one side (sometimes the right, but more 

 commonly the left, side) is commenced, and then the eye upon 

 the lower side is gradually translated to the upper side of the 

 head \ this translation being effected by an actual movement 

 of the lower eye, or by its passing through the at that time 

 soft tissues of the head, a partial twisting of the cranial bones 

 assisting to bring about the final result. When adult, both 

 eyes are situated upon one side of the head (fig. 263), and the 

 fish now keeps this side uppermost, and is dark-coloured on 

 this aspect; whilst the opposite side, on which it rests, is 

 white. From this habit of the Flat-fishes of resting upon one 

 flat surface, the sides are often looked upon as the dorsal and 

 ventral surfaces of the body. This, however, is erroneous, as 

 they are shown by the position of the paired fins to be truly 

 the lateral surfaces of the body. The mouth has its two sides 

 unequal, the pectorals are rarely of the same size, the ventrals 

 look like a continuation of the anal fin, and the branchiostegal 

 rays are six in number. 



SUB-ORDER C. ACANTHOPTERI. This sub-order is charac- 

 terised by the fact that one or more of the first rays in the 

 fins are in the form of true, unjointed, inflexible, " spiny " rays. 

 The exoskeleton consists, as a rule, of ctenoid scales. The 

 ventral fins are generally beneath or in advance of the pecto- 

 rals, and the duct of the swim-bladder is invariably obliterated. 



