Among water-animals the cetaceans may give rise to someperplexity, though they too can be rationally explained.Examples of such animals are dolphins and whales, and all othersthat have a blowhole. They have no feet, yet possess a lung thoughadmitting the sea-water. The reason for possessing a lung is thatwhich we have now stated [refrigeration]; the admission of water isnot for the purpose of refrigeration. That is effected by respiration,for they have a lung. Hence they sleep with their head out of thewater, and dolphins, at any rate, snore. Further, if they areentangled in nets they soon die of suffocation owing to lack ofrespiration, and hence they can be seen to come to the surface owingto the necessity of breathing. But, since they have to feed in thewater, they must admit it, and it is in order to discharge this thatthey all have a blow-hole; after admitting the water they expel itthrough the blow-hole as the fishes do through the gills. The positionof the blow-hole is an indication of this, for it leads to none of theorgans which are charged with blood; but it lies before the brainand thence discharges water.It is for the very same reason that molluscs and crustaceans admitwater-I mean such animals as Carabi and Carcini. For none of theseis refrigeration a necessity, for in every case they have littleheat and are bloodless, and hence are sufficiently cooled by thesurrounding water. But in feeding they admit water, and hence mustexpel it in order to prevent its being swallowed simultaneously withthe food. Thus crustaceans, like the Carcini and Carabi, dischargewater through the folds beside their shaggy parts, while cuttlefishand the polyps employ for this purpose the hollow above the head.There is, however, a more precise account of these in the History ofAnimals.Thus it has been explained that the cause of the admission of thewater is refrigeration, and the fact that animals constituted for alife in water must feed in it.
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An account must next be given of refrigeration and the manner inwhich it occurs in respiring animals and those possessed of gills.We have already said that all animals with lungs respire. The reasonwhy some creatures have this organ, and why those having it needrespiration, is that the higher animals have a greater proportion ofheat, for at the same time they must have been assigned a highersoul and they have a higher nature than plants. Hence too those withmost blood and most warmth in the lung are of greater size, and animalin which the blood in the lung is purest and most plentiful is themost erect, namely man; and the reason why he alone has his upper partdirected to the upper part of the universe is that he possesses such alung. Hence this organ as much as any other must be assigned to theessence of the animal both in man and in other cases.This then is the purpose of refrigeration. As for the constrainingand efficient cause, we must believe that it created animals likethis, just as it created many others also not of this constitution.For some have a greater proportion of earth in their composition, likeplants, and others, e.g. aquatic animals, contain a larger amount ofwater; while winged and terrestrial animals have an excess of airand fire respectively. It is always in the region proper to theelement preponderating in the scheme of their constitution that thingsexist.
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