Morever, with fishes furnished with gills, the gills in some
cases are simple in others duplicate; and the last gill in the
direction of the body is always simple. And, again, some fishes have
few gills, and others have a great number; but all alike have the same
number on both sides. Those that have the least number have one gill
on either side, and this one duplicate, like the boar-fish; others
have two on either side, one simple and the other duplicate, like
the conger and the scarus; others have four on either side, simple, as
the elops, the synagris, the muraena, and the eel; others have four,
all, with the exception of the hindmost one, in double rows, as the
wrasse, the perch, the sheat-fish, and the carp. The dog-fish have all
their gills double, five on a side; and the sword-fish has eight
double gills. So much for the number of gills as found in fishes.