Some of the lanky fishes have no fins at all, such
as the muraena, nor gills articulated like those of other fish.
And of those fish that are provided with gills, some have
coverings for this organ, whereas all the selachians have the organ
unprotected by a cover. And those fishes that have coverings or
opercula for the gills have in all cases their gills placed
sideways; whereas, among selachians, the broad ones have the gills
down below on the belly, as the torpedo and the ray, while the lanky
ones have the organ placed sideways, as is the case in all the
dog-fish.
The fishing-frog has gills placed sideways, and covered not
with a spiny operculum, as in all but the selachian fishes, but with
one of skin.
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.
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