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So still the Diodons, amphibious tribe,D. D. GARDEN dissected the amphibious creature called diodon by Linneus, and was amazed to find that it possessed both external gills and internal lungs, which he described and prepared and sent to Linneus; who thence put this animal into the order nantes of his class amphibia. He adds also, in his account of polymorpha before the class amphibia, that some of this class breathe by lungs only, and others by both lungs and gills.
With two-fold lungs the sea and air imbibe. CANT. I. l. 351
Some quadrupeds, as the beaver, water rat, and otter, are said to have the foramen ovale of the heart open, which communicates from one cavity of it to the other; and that, during their continuance under water, the blood can thus for a time circulate without passing through the lungs; but as it cannot by these means acquire oxygen either from the air or water, these creatures find it frequently necessary to rise to the surface to respire. As this foramen ovale is always open in the foetus of quadrupeds, till after its birth that it begins to respire, it has been proposed by some to keep young puppies three or four times a day for a minute or two under warm water to prevent this communication from one cavity of the heart to the other from growing up; whence it has been thought such dogs might become amphibious. It is also believed that this circumstance has existed in some divers for pearl; whose children are said to have been thus kept under water in their early infancy to enable them afterwards to succeed in their employment.
But the most frequent distinction of the amphibious animals, that live much in the water, is, that their heart consists but of one cell; and as they are pale creatures with but little blood, and that colder {19} and darker coloured, as frogs and lizards, they require less oxygen than the warmer animals with a greater quantity and more scarlet blood; and thence, though they have only lungs, they can stay long under water without great inconvenience; but are all of them, like frogs, and crocodiles, and whales, necessitated frequently to rise above the surface for air.
In this circumstance of their possessing a one-celled heart, and colder and darker blood, they approach to the state of fish; which thus appear not to acquire so much oxygen by their gills from the water as terrestrial animals do by their lungs from the atmosphere; whence it may be concluded that the gills of fish do not decompose the water which passes through them, and which contains so much more oxygen than the air, but that they only procure a small quantity of oxygen from the air which is diffused in the water; which also is further confirmed by an experiment with the air-pump, as fish soon die when put in a glass of water into the exhausted receiver, which they would not do if their gills had power to decompose the water and obtain the oxygen from it.
The lamprey, petromyzon, is put by Linneus amongst the nantes, which are defined to possess both gills and lungs. It has seven spiracula, or breathing holes, on each side of the neck, and by its more perfect lungs approaches to the serpent kind; Syst. Nat. The means by which it adheres to stones, even in rapid streams, is probably owing to a partial vacuum made by its respiring organs like sucking, and may be compared to the ingenious method by which boys are seen to lift large stones in the street, by applying to them a piece of strong moist leather with a string through the centre of it; which, when it is forcibly drawn upwards, produces a partial vacuum under it, and thus the stone is supported by the pressure of the atmosphere.
The leech, hirudo, and the remora, echeneis, adhere strongly to objects probably by a similar method. I once saw ten or twelve leeches adhere to each foot of an old horse a little above his hoofs, who was grazing in a morass, and which did not lose their hold when he moved about. The bare-legged travellers in Ceylon are said to be infested by leeches; and the sea-leech, hirudo muricata, is said {20} to adhere to fish, and the remora is said to adhere to ships in such numbers as to retard their progress.
The respiratory organ of the whale, I suppose, is pulmonary in part, as he is obliged to come frequently to the surface, whence he can be pursued after he is struck with the harpoon; and may nevertheless be in part like the gills of other fish, as he seems to draw in water when he is below the surface, and emits it again when he rises above it.