These organs are composed of two sets of elements: (1) short pyriform sense-cells terminating in a sensory hair at their outer free extremity, and a nerve-fibril at their inner; (2) supporting cells which are long, and reach the cutis, and secrete a limitans externa on their outer surface, which is pierced by the sensory hairs. These elements are grouped into eminences or ridges, which are connected in the lateral line in many instances by either modified epidermis, or by non-medullated nerve fibres. The eminences may occur on every scale of the body, e. g. in the Grey Mullet; or they may be grouped along the lateral line, while isolated eminences occur here and there on the body, e. g. in the Pike, where they are found in numbers towards the tail. In the Perch they are found only in the lateral line of the body, and in its continuation, the mucous canal system of the head. A single canal runs along the post-temporal scale. It divides, and one branch traverses the upper fork of the scale, and crosses the parietal to the opposite side of the body.
A second branch runs along the edge of the pterotic bone, and divides into a supraorbital branch which pierces the frontal bone, and runs above the nostril along the nasal bone, and an infra-orbital branch which runs along the chain of suborbital bones, and forward through the homologue of the lacrymal (?), and as a rule unites with the supra-orbital branch in front of the anterior nostril. The third main branch, the infra-maxillary, runs down the praeoperculum externally, crosses the articular, and perforates the dentary bone. The position of the sense bodies on these branches is not very evident in the Perch. There appears to be none on the connecting branch, two only on the supra-orbital, one just behind the posterior narial opening, and a second between the two apertures j four may be discovered near the free edge of the lacrymal (?), three or four on the praeoperculum, and four very evident in the dentary region. There are as a rule in the Perch no perforations corresponding to these bodies. They exist however in the Pike, and the course of the canals is therefore more easy to trace. There is much variety in fish in the arrangement, etc., of these bodies.
The canals are filled with a soft mucus, secreted by goblet-cells in the lining epithelium.
Other sense-organs - terminal or end-buds - occur on the general surface of the body, on the fins, barbules when present, lips, as well as in the mouth, and on the branchial arches. They generally project above the surface; the sensory and supporting cells are of the same length, and both alike are terminated at their free extremities by short fine points
The brain fills the brain-case only in the young fish. The space developed in the adult between the brain and the cranium is filled by a fatty arachnoid tissue. There is some doubt how far the olfactory lobes do really correspond to the structures called by the same name in higher animals. The apparently homologous parts in Lepidosteus are parts of the cerebral hemispheres, and the true olfactory lobes are very small. In some Te/eostei, e. g. the Tench, the lobes are as in many Elasmobranchii connected by a long peduncle to the brain. The cerebral hemispheres are solid. They touch the optic lobes behind, and the thalami optici are hidden from view. The pineal gland varies much in form, etc, in fish. In the Perch its basal part is conical; its middle region filamentous; its terminal part enlarged and fixed to the skull in the frontal region. The optic lobes contain large ventricles. A peculiar fold - the fornix of Gottsche - projects from the region corresponding to the valve of Vieussens in Mammalia, and divides the ventricles almost completely from the iter a 3tio ad ivtum ventriculum. The cerebellum varies much in size and shape in Te/eostei, bu it generally leaves the fourth ventricle more or less uncovered.
The sides of this ventricle are often enlarged at the roots of the fifth nerve forming trigeminal lobes, = lobi posteriores, e. g. in the Loach, Herring, and to a certain extent the Perch. Similar enlargements often occur in relation with the roots of the vagus, forming vagal lobes, e. g. in Cyprinoids. The pituitary body is composed of two parts, an anterior downward prolongation of the infundibulum, and an appended saccus vasculosus. Close to this structure, on either side, are the lobi inferiores or hypoaria, which are remarkably developed in Te/eostei, and contain ventricles communicating with the infundibulum. The optic nerves, as in all Te/eostei, cross or perforate one the other. The fifth and seventh nerves are closely united at their roots. The glossopharyngeal quits the skull by a special foramen as in Elasmobranchs, Ganoids, Dipnoi, and perenni-branchiate Amphibia. The sense-organs of the mucous canals of the head are supplied by the fifth nerve; those of the lateral line by a branch of the vagus.
The anterior narial aperture is very prominent in the Perch; it is in some fish prolonged into a tube. The posterior aperture rarely perforates the lip in Teleostei, e. g. in some Muraenoids. The folds of the mucous membrane in the Perch are, as is commonly the case, arranged in a rosette: in some instances they are parallel to one another. In the eye, the cornea is flat, the lens spherical, the retina non-vascular. There is a pecten or falciform process, highly vascular, and pigmented, which projects into the vitreous humour, and terminates in a Campanula Halleri, connected to the equator of the lens. The ear has the three typical semicircular canals. A sacculus and recessus cochleae are differentiated, and each possess a crista acustica. The aquaeductus vestibuli is closed terminally, not open as in Elasmobranchs on the surface of the head. There are generally two large otoliths, a sagitta in the sacculus, an asteriscus in the recessus cochleae.
The teeth of the Perch are fine, long, close-set, and they are anchylosed to the bone that supports them. There is much variety in the dentinal structures of the Teleostei. An external cap or tip of enamel is commonly present, but the main body of the tooth is made up of dentine in one of its three chief modifications, tubular, vaso- or osteo-dentine. Anchylosis takes place by bone developed specially in connection with each tooth. The oesophagus passes without clear distinction into the stomach, which in the Perch is of the type known as caeca/, i. e. prolonged backwards beyond the pylorus. It is siphonal, i. e. bent on itself, in the Pike. There is a well-marked pylorus, and the first (duodenal) portion of the intestine is dilated. It carries three appendices pyloricae, structures which may be absent, e. g. Pike, or present in large numbers, e. g. Salmon, where they have a linear arrangement. They are rarely united in Teleostei into a mass by connective tissue, e. g. Tunny. In the Perch, according to Krukenberg, they secrete merely mucus; in some other fish they have a pancreatic function. The loop of intestine containing the spleen is short, and there is no external mark of separation between the intestine and rectum.