On the contrary, the division into two lobes is scarcely indicated in Elasmobranchii. The region of the thalami optici is always small, and is hidden dorsally by the hemispheres and optic lobes in Teleostei. The infundibulum is large, and bears lobi inferiores (hypoaria) except in Sturgeons, Poly-pterus and Dipnoi; they are solid in Rays, hollow in other Elasmobranchii and Teleostei. It bears also a saccus vasculosus in these two orders. The pineal gland is long, filamentous, and tubular in Elasmobranchii, Acipenser, and Ceratodus, and is attached anteriorly to the cranial roof. Its size varies much in other Fish. The pituitary body is excessively large in Polypterus and Protoptems, and is sometimes pedunculate, e.g. in Lophius (the Angler), among Teleostei. The mesencephalon is undivided in Proto-pterus; in other Fish it is bilobed, and the lobes contain large ventricles into which the cerebellum projects in Teleostei. The cerebellum is small, and like that of the Frog in the Sturgeon, Polypterus and Dipnoi. It is of fair size in Teleostei; often large, lobed, and even convoluted in Elasmobranchii. It is foliate in the Tunny (Teleostei). The medulla is very long and the fourth ventricle of great extent in Elasmobranchii, Acipenser, Polypterus and Dipnoi. The restiform tracts are often convoluted anteriorly, forming trigeminal lobes in Elasmobranchii and some Teleostei, or enlarged into vagal lobes, e.g. in Cyprinoidei.
The two optic nerves cross one another in Teleostei, the right nerve supplying the left eye, the left the right eye. In other Fish there is a chiasma. The fourth (trochlear) and sixth (abducens) nerves appear to be absent in Dipnoi. The facial and auditory have a common root, as in many Amphibia and Reptilia. The glossopharyngeus is a separate nerve, with a separate foramen in the skull. The vagus has several roots (five at least) in Elasmobranchii and Dipnoi; in the latter and some of the former independent ventral roots as well. There is no spinal accessory, and the hypoglossus is represented by spinal nerves. The fifth, the facial in Teleostei, the glossopharyngeus and vagus have intracranial dorsal branches; that of the fifth in Teleostei extends down the body near the median dorsal line; it is connected both to the spinal nerves and the lateral line branch of the vagus, and supplies the muscles and integument of the dorsal fins. The lateral line branch of the vagus lies beneath the integument, except in Elasmobranchii, where it is deeply placed in the septum, between the dorso- and ventro-lateral muscles.
The anterior roots of each spinal nerve pass in Elasmobranchii through or behind the neural arches, the posterior roots through or behind the intercalary pieces; they therefore alternate in position. In Acipenser the roots of the spinal nerves of one side of the body alternate in position with the roots of the other side. The spinal ganglia in Teleostei sometimes lie within the spinal canal, a most unusual position. The anterior end of the sympathetic system is connected to the vagus in Elasmobranchii and chondrostean Ganoidei, but in Teleostei to the third nerve, as in higher Vertebrata.
Tactile barbules are found on the head of the Sturgeon, the Cods, many Siluroidei. The nasal sacs are placed ventrally in front of or near the angles of the mouth in Elasmobranchii and Holocephali, on the dorsal aspect of the face in front of the eyes in Ganoidei and Teleostei. They have usually an anterior and posterior opening, the former being occasionally, as in Polypterus and some Teleostei, prolonged into a tube. A channel runs down to the angle of the mouth from the nasal chamber in Rays and some Sharks. In Dipnoi the anterior aperture lies just in front of the mouth and outside it, the posterior within it externally to the vomerine teeth. The mucous membrane is disposed in folds. Certain Plectognathi (Tetrodon) are said to have solid olfactory tentacles. Protopterus alone has an incomplete fenestrated nasal capsule. The eyes have no. glands, and lids are represented by slight folds, most marked in Elasmobranchii, some of which, e. g. Mustelus, Carcharias, have a third eyelid or nictitating membrane. The bulb is imbedded in a fatty gelatinous tissue traversed by connective tissue fibres. It is small in Dipnoi, rudimentary in some deep-sea, cave- or mud-inhabiting Teleostei, but is usually large.
In Pleuro-nectidae (Soles) both eyes come to be placed on the same side of the head. The cornea is flat, the sclerotic generally strengthened by calcifications. An argentea - a silvery or golden cellular layer, the colour due to minute crystals - lies externally to the choroid in most Teleostei, and extends into the iris. The latter is absent in Dipnoi', its musculature is feebly developed. The ciliary muscle is perhaps wanting. The lens is spheroidal, and almost touches the cornea. A falciform process is present in most Teleostei; it ends in a bulbus expansion, the Campanula Halleri, which is attached at the equator of the lens. It is pigmented, vascular, and supplied with nerves; the Campanula contains muscular tissue, and is probably-concerned in accommodation. A choroid gland or bipolar vascular rete is found in most Teleostei close to the optic nerve, and lying between the argentea and the choroid coat. It receives the blood of the ophthalmic artery coming from the pseudobranchia, and is supposed to represent the spiracular gill. The auditory structures are completely inclosed in cartilage in Elasmobranchii and Dipnoi, but in Holocephali, Ganoidei, and Teleostei the inner, i. e. cranial, wall is membranous.
The aquaeductus vestibuli opens externally on the head in most Elasmobranchii. The connection between the vestibule and saccule is closed in Batoidei, bony Ganoidei, and some Teleostei) and in the latter the cochlear outgrowth (= lagena) is large. It contains a small otolith, the asteriscus, whilst the saccule has one of great size, the sagitta. Among Teleostei in some Acanthopteri (Percoids and Sparoids) a process of the air-bladder is applied to a membranous fenestra in the auditory capsule; in the Clupeidae a similar process enters the capsule and comes into relation with the vestibule, whilst in Siluroidei and Cyprinoidei a chain of bones derived from the vertebral centra and ribs connects the air-bladder to the vestibule.