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.