The spinous armature of the opercle and the two spines of the first dorsal fin constitute the weapons.
The tip of its opercle is prolonged like an ear and jet black in color, with a dash of bright scarlet along its lower edge.
In most of the higher fishes a small accessory gill (pseudobranchia) is developed in the skin of the inner side of the opercle.
On our sunfish we see that the opercle ends behind in a long and narrow flap, which looks like an ear.
In Erpetoichthys only the opercle is distinct among the gill-covers.
The largest uppermost flat bone of the gill-covers is known as the opercle .
Overlying these is the system of membrane bones already mentioned, including the opercle or gill-cover.