PARTS OF ANIMALS, IV. xii.-xiii. 



the weight of its body tends forward less than that of 

 other birds. 



All birds have testicles, but they are inside the 

 body. The reason for this will be stated in the 

 treatise on the different methods of generation 

 among animals.^ 



This concludes our description of the parts of Birds, (iii.) Fishes ; 



XIII. In the tribe of Fishes the external parts 

 are still further stunted. Fishes have neither legs, 

 hands, nor wings (the reason has been stated earlier), 

 but the whole trunk has an uninterrupted line from 

 head to tail. Not all fishes' tails are alike ; but the Tail, 

 general run of them have similar tails, though some 

 of the flat-fish have a long, spiny one, because the 

 material for the tail's growth goes into the width 

 of the fiat body : this happens in the torpedo-fishes, 

 in the Trygons, and any other Selachians of the same 

 sort. These have long, spiny tails. Others have 

 short, fleshy ones, and for the selfsame reason : it 

 comes to the same thing whether the tail is short 

 and has a good deal of flesh or long with little flesh. 



In the fishing-frog ^ the opposite has taken place. 

 Here, the wide, flat part of the body in front is not 

 fleshy ; Nature has taken the fleshy material away 

 from the front and added an equivalent amount at 

 the back — in the tail. 



Fishes have no separate limbs attached to the body. 

 (a) This is because Nature never makes anything that 

 is superfluous or needless, and by their essence and 

 constitution^ fishes are naturally swimmers and so 



^ Lophius piscatorius, known as the " goosefish " in 

 U.S.A., erroneously inchided by Aristotle {De gen. an. 

 754 a 25) with the Selachia, though he observed that it 

 diflFered in many important points. 



" Logos : see Introduction, pp. 2Q f. 



417 



