Figure 1. The hearing, lateral line and vestibular systems of a fish. (a) Dorsal view of a generalized teleost fish illustrating the position of the inner ear (ie) and the swim bladder (sb). Changes in pressure, such as the passing of a propagated sound wave, will affect changes in swim bladder volume. Changes in swim bladder volume will be transmitted to the inner ear, either directly or via bony ossicles. (b) A generalized inner ear. Three semicircular canals are present and the sensory maculae are indicated by black ovals. (c) The mechanics of semicircular canal function. A ridge capped by hair cells (cr, crista) sits on the floor of the ampulla (a). The apical projections of the hair cells are embedded in a gelatinous cupula (cu). As an angular acceleration is applied to the canal (solid arrows), fluid flows in the opposite direction (dashed arrows). Internal fluid flow distorts the elastic cupula (arrowhead), which is attached to both the crista and the roof of the ampulla. Distortion of the cupula creates a shearing force at the apex of the hair cells. (d) The mechanics of otolith organ function. The apical projections of the macular hair cells are embedded in the otolith (o). A linear acceleration applied to the head will move the macula (dark arrow), but the greater inertia of the more massive otolith will cause it to lag behind, applying a shearing force to the apical surface of the hair cells (light arrow). (e) Lateral view of the head of a generalized cyprinid teleost. Both lateral line canals and superficial neuromasts are present. The canals are shown as thin tubes that open to the surface through pores (open circles). In the canal, a neuromast is present between each pair of pores (large grey ellipses). Superficially, groups of freestanding neuromasts are present, each with a discrete orientation (black ellipses; the major axis of each ellipse signifies the axis of best sensitivity of that neuromast). (f) The mechanics of superficial neuromast function. Any flow past the surface of the neuromast will deflect the cupula (arrow). (g) The mechanics of canal neuromast function. A difference of pressure at adjacent canals (indicated by + or –) will cause canal fluid to accelerate, deflecting the cupula (arrow). a, ampulla; cp, canal pore; cr, crista; cu, cupula; ie, inner ear; L, lagena; N, macula; nm, neuromast; o, otolith; S, saccule; sb, swim bladder; U, utricle. (Panels (a)–(d) redrawn from Platt et al., 1989.)

Figure 2. The electrosensory system of weakly electric teleost fishes. (a) Lateral view of a weakly electric fish, Gymnarchus, illustrating the position of the organs used to produce an electrical field (dashed lines). (b) Dorsal view of Gymnarchus and the electrical field that it generates. Current flows between the tail, which contains the electrogenic organs, and the head, which contains the electrosensory organs. A perfectly conducting object (shaded circle) will not distort the flow of the electric field. Inset: An object that does not conduct as well as water (shaded circle) will distort the electric field, providing the fish with information about the object's size and distance. (c) An ampullary electric organ, the duct of which opens into the water surrounding the weakly electric fish. The arrow indicates an electrosensory receptor cell. (d) A tuberous electric organ, the duct of which is covered by the surface of the animal's skin. The arrow indicates an electrosensory receptor cell. SN, sensory nerve. (Panels (a) and (b) based on Lissman, 1963; (c) and (d) based on Szabo, 1974.)

Figure 3. The specialized heat-sensing organs of snakes. (a) The head of a rattlesnake, Crotalus durissus, illustrating the position of the pit organ, which is depicted as a black circle and indicated by an arrow. (b) The head of a boa, Boa constrictor, illustrating the position of the heat-sensing organs, depicted as black circles, along the upper and lower margins of the jaw. One organ is marked with an arrow. (c) The head of a python, Python reticulatus, illustrating the position of the heat sensing organs, depicted as black circles, located in grooves along the upper and lower margins of the jaw. One organ is marked with an arrow. (d) Cross-section through the pit organ of a rattlesnake, illustrating the position of the heat-sensitive membrane that spans the pit. The trigeminal nerve innervates the membrane, with free nerve endings terminating in swellings (open circles). (e) The heat-sensing organ of a boa. The surface of the scale is not visibly specialized, but is heavily innervated by the trigeminal nerve. (f) The heat-sensing organ of a python, which appears as a groove in a scale and is innervated by endings of the trigeminal nerve. EP, epidermis; IC, inner chamber of the pit organ; M, membrane; NF, nerve fibres; OC, outer chamber of the pit organ. (Based on von Düring and Miller, 1979.)