Chordates are a group of deuterostomes that show a number of complex adaptations. They have four major characters that distinguish them from other deuterostomes: a notochord, a dorsal nerve cord, pharyngeal slits, and a post-anal tail.
The dorsal, hollow nerve cord is basically a sheet of ectoderm rolled into a tube. A chordate's nerve cord develops into the brain and spinal cord in adults.
Just below the nerve cord is the notochord, which exists in all chordate embryos, and some adults. The notochord is a long flexible rod that provides support. In vertebrates, it becomes part of the intervertebral discs in adults. Typically the muscles surrounding the notochord are arranged in distinct segments.
The pharyngeal slits are openings in the pharynx (part of the throat) of an animal. In the most primitive state these slits allow water to enter the mouth and exit without passing through the digestive system. In the more derived state the pharyngeal slits (and the tissue between them, the pharyngeal arches) serve various functions, ranging from gas exchange to food collection.
At some point in their development, all chordates have a tail that extends beyond the anus (a postanal tail). As with all of these basic chordate characteristics, the tail can be present in the developing embryo and absent in the adult (depending on the type of organism).
There are three subphyla within Phylum Chordata: Vertebrata, Cephalochordata, and Uroochordata. All cephalochordates and urochordates are invertebrates (animals without backbones), as were all of the animals discussed in the previous tutorials. All members of the Subphylum Vertebrata are vertebrates (animals with backbones).
Subphylum Cephalochordata includes the cephalochordates (lancelets), which are small marine animals that burrow, tail first, into the sand. In this position they are able to draw food particles into their mouths via their waving tentacles. These animals are classified as chordates because they have all four of the chordate characteristics and, unlike the other chordate groups, they retain all four characteristics through adulthood.
Subphylum Urochordata includes the tunicates ("sea squirts"), which are marine animals that are typically sessile as adults. The lightbulb tunicates in the figure above are adhering to coral.
The adult tunicate does not appear to possess most of the standard chordate characteristics. However, the larval stage clearly exhibits chordate characteristics; note the notochord, the hollow nerve cord, the pharyngeal gill slits, and the post-anal tail. In adults the gill slits are modified for filter feeding. Molecular evidence supports the idea that the urochordates are the group most closely related to the vertebrates.