PARTS OF ANIMALS, IV. xiii. 



have any other footless animals, and this includes the 

 serpents. In fish the passage for the residue and for 

 the generative secretion is one and the same ; and 

 this is so in all other oviparous animals, four-footed 

 ones included. This is because they have no bladder 

 and produce no liquid residue. 



Thus we have seen what are the differences to be inter- 

 noticed in fish as a group as compared wdth other "Ltures : 

 animals. Dolphins and whales and all such Cetacea, ^'•) Cetacea. 

 however, have no gills, but they have a blowhole 

 because they have a lung. They cannot help letting 

 the sea-water enter the mouth because they feed in 

 the water, and once it has got in they must get it out 

 again, and they do so through the blowhole. Gills, 

 of course, are of service herein to those creatures that 

 do not breathe. The reason for this has been given 

 in my book on Respiration'^ '. no creature can breathe 

 and at the same time have gills ; instead, these 

 Cetacea have a blowhole for getting rid of the water. 

 It is placed in front of the brain, otherwise it would 

 separate the brain from the spine. The reason why 

 these creatures have a lung and breathe is that large 

 animals need more heat than others to enable them to 

 move ; consequently they have a lung inside them ^ 

 full of heat derived from the blood. They are, in a 

 way, land-animals as well as water-animals : they 

 inhale the air, hke land-animals, but they have no 

 feet and they get their food from the water as water- 

 animals do. Similarly, seals and bats are in an inter- (ii.) Seaia 

 mediate position. Seals are between land-animals ^""^ ^**** 

 and water-animals, bats between land-animals and 

 fliers : thus they belong to both classes or to neither. 

 " References given above, see on 696 b 2, 



427 



