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The head or skull includes the skull roof (a set of bones covering the brain, eyes and nostrils), the snout (from the eye to the forward-most 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 preopercle. The operculum and preopercle may or may not have spines.
The brain of the lamprey is divided into the telencephalon, diencephalon, midbrain, cerebellum, and medulla. [22] Lampreys have been described as the only living vertebrates to have four eyes, [23] having a single pair of regular eyes as well as two parietal eyes: a pineal and parapineal one (the exception is members of Mordacia). [24]
Each rhopalium carries six eyes of four morphological types (lower lens eye LLE, upper lens eye ULE, pit eye PE and slit eye SE) and a light sensitive neuropil (NP, red broken line). The eyes are responsible for the image formation in the animal and the light sensitive neuropil is thought to be involved in diurnal activity".
In sharks and some primitive bony fish, a spiracle, a small extra gill opening, is found behind each eye. [1] The skull in fishes is formed from a series of only loosely connected bones. Jawless fish and sharks possess only a cartilaginous endocranium, with both the upper and lower jaws being separate elements.
Their eyes are small and may not function at all. [41] [42] At the very bottom of the ocean flatfish can be found. Flatfish are benthic fish with a negative buoyancy so they can rest on the seafloor. Although flatfish are bottom dwellers, they are not usually deep sea fish, but are found mainly in estuaries and on the continental shelf.
Fish physiology is the scientific study of how the component parts of fish function together in the living fish. [2] It can be contrasted with fish anatomy, which is the study of the form or morphology of fishes. In practice, fish anatomy and physiology complement each other, the former dealing with the structure of a fish, its organs or ...
In many lower vertebrates (such as species of fish, amphibians and lizards), the pineal gland is associated with parietal or pineal eye. In these animals, the parietal eye acts as a photoreceptor, and hence are also known as the third eye, and they can be seen on top of the head in some species. [51]
Diagrammatic vertical section through the eye of teleost fish. Fish have a refractive index gradient within the lens which compensates for spherical aberration. [4] Unlike humans, most fish adjust focus by moving the lens closer or further from the retina. [5] Teleosts do so by contracting the retractor lentis muscle.