Superclass Pisces

Superclass Pisces includes the true fishes — aquatic, gill-breathing vertebrates with fins. They are poikilothermic ('cold-blooded'), meaning their body temperature varies with the surrounding water temperature. A lateral line system runs along the body, containing rheoreceptors that detect water currents. Locomotion is by body muscles together with fins; the caudal (tail) fin functions like a steering wheel.

The exoskeleton consists of dermal scales, and the endoskeleton may be bony or cartilaginous, depending on the class. The streamlined, boat-shaped body offers minimal resistance while swimming. Respiration occurs through gills. The heart is two-chambered and ventrally placed, providing single, closed circulation; because it always carries deoxygenated blood on its way to the gills, it is described as a venous heart. Fish have a well-developed brain with prominent olfactory lobes. Sexes are separate, and mos …