30 Vertebrate 



mouth is also usually awry, so as to give the greatest 

 amount of facility of swallowing consistent with 

 position. As in this distorted position the dorsal 

 and anal fins are disposed as lateral fringes, they 

 functionally replace the paired fins as directors of 

 motion, and hence the pectoral and ventral fins are 

 usually small or deficient. One interesting feature in 

 these fishes is that their embryos at a very early stage 

 are perfectly symmetrical, and gradually develope the 

 one-sided torsion as growth progresses, the displaced 

 eye having been traced by observers in its curious 

 pilgrimage around the front of the obliquely growing 

 head from the under to the upper side. The turbot, 

 plaice, flounder, sole, dab, and fluke are well-known 

 examples ; the largest species inhabiting our seas is 

 the halibut, which sometimes has been known to 

 attain the weight of over 500 pounds. Another 

 curious point is noteworthy viz. that though in each 

 genus the side to which the eyes are displaced is 

 usually constant, yet erratic reversed examples are 

 occasionally met with. Thus while in the flounder 

 and plaice the eyes are usually on the right, in such 

 reversed cases they are found looking to the left ; 

 such abnormalities are easily understood by the light 

 of the embryonic development of the group. 



One genus of fish of this sub-order, named 

 Fierasfer, is parasitic within the bodies of certain 

 sea-cucumbers, or holothurians, and star-fishes, and 

 is found in the Indian Ocean. 



SUB -ORDER 3, ACANTHOPTERI. Spiny -finned 

 fishes with a ductless swimming-bladder, or else none. 

 This is the most numerous and most specialised group 



