Nervous system and sense organs of fishes
A fish’s central nervous system consists of the brain and spinal cord; the brain has five parts: forebrain, diencephalon, midbrain, cerebellum and medulla. The cerebellum controls movement, and the medulla controls breathing, circulation and digestion. The eye has no lids, sits on the sides of the head, sees close objects and distinguishes shape and colour. The nostrils open into sacs with smell cells. The hearing organ is the inner ear, and water carries sound well. Taste cells are in the mouth and on the body surface, and the carp’s barbels are touch organs. The lateral line senses water current, pressure and obstacles: even a blind fish avoids obstacles. A response to a stimulus is a reflex: the stimulus travels along sensory fibres to the brain and from there to muscles. Inborn reflexes are unconditioned reflexes (instincts) and are inherited. Reflexes formed during life are conditioned: for example, if food is given with a light on, fish gather at the signal; they are not inherited and fade away if the signal is no longer followed by food.
Experiment (at home, with adults): give the same signal before feeding aquarium fish and record their reaction for 5 days. Safety: adults handle aquarium electrical equipment; never touch a socket with wet hands; do not overfeed the fish.