Chordata - Vertebrates and Other Animals

Facts about the Chordates

Colonial Tunicate / NOAA
Orange colonial tunicate, Gray's Reef National Marine Sanctuary, Georgia. This tunicate is in the Phylum Chordata. NOAA

The Phylum Chordata contains some of the most familiar animals in the world, including humans. What sets them apart is that they all have a notocord, or nerve cord, at some stage of development. You might be surprised by some other animals in this phylum, as they are very different from humans, birds, fish and the fuzzy animals that we usually think of when we think of the Phylum Chordata.

Chordates Have Backbones or Notocords

Animals in the Phylum Chordata may not all have a spine (some do, which would classify them as a vertebrate animal), but they all have a notochord.

The notochord is like a primitive backbone, and is present at least at some stage of their development. These may be seen in early development, and in some they develop into other structures before birth:

  • All have a tubular nerve cord (such as the spinal cord) above the notocord, which is gelatin-like and encased in a tough membrane.
  • All have gill slits that lead into the throat or pharynx.
  • All have blood enclosed in blood vessels, although they may not have blood cells.
  • All have a tail that has no internal organs and extends beyond the backbone and anus.

Three Types of Chordates

While animals like humans, mammals and birds are all vertebrates in the Phylum Chordata, not all animals in the Phylum Chordata are vertebrates. The Phylum Chordata contains three Subphyla.

  • The Vertebrates: Subphylum Vertebrata. When you think of animals, you probably are thinking about the vertebrates. These include all of the mammals, reptiles, birds, amphibians, and most fish. In vertebrates, a backbone develops around the notocord, made of bone or cartilage separated into segments called vertebrae. It protects the spinal cord. There are over 57,000 species of vertebrates.
  • The Tunicates: Subphylum Tunicata. These include the salps, larvaceans, and tunicates such as the sea squirt. They are invertebrates as they don't have a backbone, but they do have a notochord during development. They are marine filter-feeders, with some tunicates living attached to rocks for most of their life except for a free-swimming larval stage. The salps and larvaceans are tiny, plankton-like free-swimming animals, although the salps spend a generation as an aggregate chain. They have a very primitive nervous system. Many think that the ancestors of the tunicates also evolved into the vertebrates. There are about 3,000 species of tunicates.
  • The Cephalochordates: Subphylum Cephalochordata. There are only about 30 species in this grouping. They include the lancelets, which are small aquatic filter-feeders that are fishlike. They have a large notochord and a primitive brain. Their circulatory system doesn't have a heart or blood cells.

Classification of the Chordates

Kingdom: Animalia

Phylum: Chordata

Classes (the classes in bold below include marine species):

Subphylum Tunicata (formerly Urochordata)

Subphylum Cephalochordata

  • Cephalochordata (lancelets)

Subphylum Vertebrata