ious 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.

Some amphibious 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 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 the