What is Subphylum Vertebrata explain?

This large group of animals takes its name from its distinguishing feature: the vertebral column, or the backbone. The backbone made of segmented vertebrae is hollow, and surrounds the nerve, or spinal cord. Other characteristics include a well-developed head and brain, an internal circulatory system, two pairs of limbs, gills or lungs, and separate sexes.

There are seven living Classes of Vertebrates, three of which are collectively referred to as fishes: Class Agnatha (jawless vertebrates), Class Chondrichthyes (cartilaginous sharks and rays), and Class Osteichthyes (bony fishes). The remaining four classes all have two pairs of appendages for use on land, and therefore are referred to as tetrapods. "Tetra" in Greek means four, and "poda" refers to feet. The four classes of tetrapods are the familiar frogs (Class Amphibia), reptiles (Class Reptilia), birds (Class Aves), and the mammals (Class Mammalia).