Most fish are cold-blooded, limbless, aquatic, vertebrate animals with a glandular secretion that decreases friction. Most fish have scales. A typical fish is torpedo-shaped, with a head containing a brain and sensory organs, a muscular-walled trunk with a cavity containing internal organs, and a muscular tail. Most fish propel themselves through water with weaving movements and control direction with their fins. Although some fish, such as sharks, bear live young, most fish eggs are fertilized and hatched in water. There are over 20,000 species of fish in three classes: Agnatha, the most primitive fishes, jawless and without paired pelvic and pectoral fins (such as the lamprey); Chondrichthyes, cartilaginous fishes, with skeletons of cartilage but no swim bladder or lungs (such as the shark and ray); and Osteichthyes, bony fishes, the most highly developed. Bony fishes have a bony skeleton and a swim bladder or lungs, and are divided into fleshy-finned (such as the lobefin and lungfish) and ray-finned fishes (such as BASS). The largest fish is the whale shark (Rhincodon typus), reaching 50 feet (15 meters) in length and are found worldwide in tropical seas. The smallest is the 1/2 inch (1.3 centimeter) goby (family Gobiidae) of the Philippines.
