On the other hand, the gill slits are a characteristic feature to cartilaginous fish such as sharks and rays, in which they are characterized by the absence of a single outer gill cover
and replace by individual openings to gills (Gerhart et al., 2005).
A vivid black beauty mark with a reddish, crescent border flares out from the trailing edge of its gill cover
One notable marking is the blue or black "ear," which is actually an extension of the gill cover
. On the gill rakers (tooth-like structures on the inner edge of the gill arches), bluegill have a bright blue edging.
You usually don't need to pinch your thumb on the outside of the gill cover
. Walleyes rarely thrash wildly when held this way.
These appetitive components include saturation of body coloration, erection of the opercles, or gill cover
, orientation and movement characteristics (Simpson, 1968).
However, the fossils don't include an operculum, or gill cover
, that would have protected the delicate structures and moved oxygenated water across them.