On Youth And Old Age, On Life And Death, On Breathing   

have no lung at all or have a bloodless one require less

refrigeration.

16



Concerning the bloodless animals we have declared that in some cases

it is the surrounding air, in others fluid, that aids the

maintenance of life. But in the case of animals possessing blood and

heart, all which have a lung admit the air and produce the cooling

effect by breathing in and out. All animals have a lung that are

viviparous and are so internally, not externally merely (the

Selachia are viviparous, but not internally), and of the oviparous

class those that have wings, e.g. birds, and those with scales, e.g.

tortoises, lizards, and snakes. The former class have a lung charged

with blood, but in the most part of the latter it is spongy. Hence

they employ respiration more sparingly as already said. The function

is found also in all that frequent and pass their life in the water,

e.g. the class of water-snakes and frogs and crocodiles and hemydes,

both sea- and land-tortoises, and seals.

All these and similar animals both bring forth on land and sleep

on shore or, when they do so in the water, keep the head above the

surface in order to respire. But all with gills produce

refrigeration by taking in water; the Selachia and all other

footless animals have gills. Fish are footless, and the limbs they

have get their name (pterugion) from their similarity to wings

(pterux). But of those with feet one only, so far as observed, has

gills. It is called the tadpole.

No animal yet has been seen to possess both lungs and gills, and the

reason for this is that the lung is designed for the purpose of

refrigeration by means of the air (it seems to have derived its name

(pneumon) from its function as a receptacle of the breath (pneuma)),

while gills are relevant to refrigeration by water. Now for one

purpose one organ is adapted and one single means of refrigeration