The Chordates - An Introduction

Fish, Amphibians, reptiles, birds and mammals are all very different but are all members of the phylum Chordata.
Members of the phylum Chordata are called chordates. To be classified as a chordate and animals hash to have four key characteristics.
A chordate is an animal that has or had for some stage of life, a dorsal, hallow nerve cord; a notochord; pharyngeal pouches and a tail that extends beyond the anus.
The hollow nerve cord runs along the dorsal (back) part of the body. Nerves branch from this cord at regular intervals and connect to the internal organs, muscles, and sense organs.
The notochord is a long supporting rod that runs through the body just below the nerve cord. Most chordates have a notochord only when they are embryos
Pharyngeal pouches - These are paired structures in the throat (pharynx) region. In some chordates - such as fish and amphibians - slits develop that connect the pharyngeal pouches to the outside of the body. These slits may then develop into gills that are used for gas exchange.

At some point in their lives, all chordates have a tail that extends beyond the anus. The tail can contain bone and muscle and is used in swimming by many aquatic species.

Most Chordates are vertebrates. More than 99% of them are in the subphylum Vertebrata and are called vertebrates. A vertebrate is a chordate that has a strong supporting structure called a backbone. IN vertebrates, the dorsal, (hollow nerve cord) is called the spinal cord. The front end is in the brain. It IS the backbone and replaces the notochord in most developing vertebrates and is made of small individual segments called vertebra.

However, there are SOME nonvertebrate chordates. The two groups of nonvertebrate chordates are tunicates and lancelets. Both are soft-bodies marine organisms. They have a hollow nerve cord, a notochord, pharyngeal pouches, and a tail.
Tunicates are filter-feeding organisms that look a bit like tubes. Lancelets are small and fish-like creatures that live on the sandy ocean bottom.