Aristotle On Youth and Old Age,On Life and Death,On Breathing-Part 16

Concerning the bloodless animals we have declared that in some casesit is the surrounding air, in others fluid, that aids themaintenance of life. But in the case of animals possessing blood andheart, all which have a lung admit the air and produce the coolingeffect by breathing in and out. All animals have a lung that areviviparous and are so internally, not externally merely (theSelachia are viviparous, but not internally), and of the oviparousclass those that have wings, e.g. birds, and those with scales, e.g.tortoises, lizards, and snakes. The former class have a lung chargedwith blood, but in the most part of the latter it is spongy. Hencethey employ respiration more sparingly as already said. The functionis 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 sleepon shore or, when they do so in the water, keep the head above thesurface in order to respire. But all with gills producerefrigeration by taking in water; the Selachia and all otherfootless animals have gills. Fish are footless, and the limbs theyhave get their name (pterugion) from their similarity to wings(pterux). But of those with feet one only, so far as observed, hasgills. It is called the tadpole.No animal yet has been seen to possess both lungs and gills, and thereason for this is that the lung is designed for the purpose ofrefrigeration 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 onepurpose one organ is adapted and one single means of refrigerationis sufficient in every case. Hence, since we see that Nature doesnothing in vain, and if there were two organs one would bepurposeless, this is the reason why some animals have gills, otherslungs, but none possess both.