From these subphyla tunicates (urochrdates) and lancelets (cephalochordates) are the simplest and the most primitives forms of chordates. The vertebrates, however, are the most advanced and the higher chordates.

One of the divisions of the chordates is the vertebrates. The vertebrates are distinguished from other chordates in having.

• a backbone, or vertebral column, that replaces the notochord, the vertebrae, these are cartilaginous or bony segments on the vertebral column.
• the cranium, or a braincase found anterior to the vertebral column. The cranium enclosed and protects the brain, the enlarged anterior end of the nerve cord.
• a true brain or cephalization, i.e. concentration of nerve cells and sense organs in a defined head.

All vertebrates, however, share certain their characteristics with other groups. Among these characteristics some are the following:

• a closed circulatory system with a two, three or four chambered ventral heart.
• paired – kidneys.
• a complete digestive tract and large digestive glands (liver and pancreas).
• muscles attached to the skeleton for movement.