Vertebrate, << VUR tuh briht or VUR tuh brayt, >> is an animal with a spinal column (backbone) and a cranium (brain case). Tens of thousands of species (kinds) of vertebrates have been named and described. They may be divided into eight classes: (1) hagfishes (Myxini); (2) lampreys (Cephalaspidomorphi); (3) sharks and other cartilaginous fish (Chondrichthyes); (4) bony fish (Osteichthyes); (5) frogs and other amphibians (Amphibia); (6) reptiles (Reptilia); (7) birds (Aves); and (8) mammals (Mammalia).
Most vertebrates have a spinal column made of bones called vertebrae (see Spine). But some, such as the shark, do not have a bony spinal column. Sharks have vertebrae made of cartilage (waxy tissue).
All vertebrates are bilaterally symmetrical—that is, the left and right sides of the body are mirror images of each other. The body is usually divided into a head and a trunk. The more advanced land vertebrates have a neck. In mammals (animals that produce milk to feed their young), the trunk is divided into a thorax (chest) and abdomen. Vertebrates never have more than two pairs of limbs.
