Understanding the Phylum Chordata

Facts About the Chordates

Tyrannosaurus Rex at AMNH
Mark Ryan/Flickr/CC BY-ND 2.0

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 ​notochord—or nerve cord—at some stage of development. You might be surprised by some other animals in this phylum, as there are more than the humans, birds, fish, and fuzzy animals that we usually think of when we think of the phylum Chordata.

All Chordates Have Notochords

Animals in the phylum Chordata may not all have a spine (some do, which would additionally classify them as vertebrate animals), but they do all have a notochord. The notochord is like a primitive backbone, and it is present at at least some stage of development. These may be seen in early development—in some species they develop into other structures even before birth.

Phylum Chordata Facts

  • All have a tubular nerve cord (such as the spinal cord) above the notochord, 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 contains no internal organs and extends beyond the backbone and anus.

3 Types of Chordates

While some animals in the phylum Chordata are vertebrates (e.g. humans, mammals, and birds), not all animals are. The phylum Chordata contains three subphyla:

  • The vertebrates (subphylum Vertebrata): When you think of animals, you probably think about the vertebrates. These include all mammals, reptiles, birds, amphibians, and most fish too. In vertebrates, a backbone develops around the notochord; it is made of bone or cartilage separated into segments called vertebrae, and its primary purpose is to protect 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 lives 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. In general, members of the subphylum Tunicata have very primitive nervous systems, and many taxonomists think that their ancestors also evolved into vertebrates. There are about 3,000 species of tunicates.
  • The cephalochordates (subphylum Cephalochordata): This subphylum includes the lancelets, which are small aquatic filter-feeders that are fish-like. Members of the subphylum Cephalochordata have large notochords and primitive brains, and their circulatory systems have neither heart nor blood cells. There are only about 30 species in this grouping.

Classification of the Chordates

Kingdom: Animalia

Phylum: Chordata

Classes:

Subphylum Vertebrata

Subphylum Tunicata (formerly Urochordata)

Subphylum Cephalochordata

  • Cephalochordata (lancelets)