The head of a fish is the part of the body from the beginning of the mouth to the back end of the gill cover.
The head of a fish includes the snout, from the eye to the frontmost point of the upper jaw, the operculum or gill cover (absent in sharks and jawless fish), and the cheek, which extends from the eye to the preoperclearance. The operculum and preoperculum may or may not have spines. Sharks and some primitive bony fishes have a spiracle behind each eye, a small additional gill opening. The skull in fish is formed from a series of only loosely connected bones. Jawless fish and sharks possess only a cartilaginous endocranium, with the upper and lower jaws being separate elements. Bony fishes have an additional dermal bone that forms a more or less cohesive skull roof in bivalves and bachelor fishes. The lower jaw defines the chin.
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