is sufficient in every case. Hence, since we see that Nature does

nothing in vain, and if there were two organs one would be

purposeless, this is the reason why some animals have gills, others

lungs, but none possess both.



17



Every animal in order to exist requires nutriment, in order to

prevent itself from dying, refrigeration; and so Nature employs the

same organ for both purposes. For, as in some cases the tongue

serves both for discerning tastes and for speech, so in animals with

lungs the mouth is employed both in working up the food and in the

passage of the breath outwards and inwards. In lungless and

non-respiring animals it is employed in working up the food, while

in those of them that require refrigeration it is the gills that are

created for this purpose.

We shall state further on how it is that these organs have the

faculty of producing refrigeration. But to prevent their food from

impeding these operations there is a similar contrivance in the

respiring animals and in those that admit water. At the moment of

respiration they do not take in food, for otherwise suffocation

results owing to the food, whether liquid or dry, slipping in

through the windpipe and lying on the lung. The windpipe is situated

before the oesophagus, through which food passes into what is called

the stomach, but in quadrupeds which are sanguineous there is, as it

were, a lid over the windpipe-the epiglottis. In birds and oviparous

quadrupeds this covering is absent, but its office is discharged by

a contraction of the windpipe. The latter class contract the

windpipe when swallowing their food; the former close down the

epiglottis. When the food has passed, the epiglottis is in the one

case raised, and in the other the windpipe is expanded, and the air

enters to effect refrigeration. In animals with gills the water is