No fish is furnished with testicles, nor any other creature
that has gills, nor any serpent whatever: nor, in short, any animal
devoid of feet, save such only as are viviparous within themselves.
Birds are furnished with testicles, but these are internally situated,
close to the loin. The case is similar with oviparous quadrupeds, such
as the lizard, the tortoise and the crocodile; and among the
viviparous animals this peculiarity is found in the hedgehog. Others
among those creatures that have the organ internally situated have
it close to the belly, as is the case with the dolphin amongst animals
devoid of feet, and with the elephant among viviparous quadrupeds.
In other cases these organs are externally conspicuous.

We have already alluded to the diversities observed in the
attachment of these organs to the belly and the adjacent region; in
other words, we have stated that in some cases the testicles are
tightly fastened back, as in the pig and its allies, and that in
others they are freely suspended, as in man.