All members of the phylum Chordata exhibit the following four major morphological features at some stage of their life cycle.

1. Notochord
2. Dorsal Hollow Nerve Cord (DHNC)
3. Pharyngeal Slits
4. Post-anal Tail

The notochord, the feature for which the subphylum is named, is an elastic, rod-like structure that runs parallel to and between the digestive tract and central nervous system. It is found in all chordate embryos and serves as a support structure, resisting "telescoping" of the body when longitudinal muscles shorten. The notochord is composed of fluid-filled cells wrapped in a fibrous sheath. Although functionally replaced by the vertebral column in many chordate groups (principally the "higher" vertebrates), the notochord is still an important embryonic structure in all chordates. In adult humans, and other mammals, the notochord is preserved as the nucleus pulposus, a small gel-like region at the core of each of the intervertebral discs between adjacent vertebrae.

The dorsal, hollow nerve cord forms the central nervous system (brain and spinal cord) of all chordates. It is distinct from the central nervous systems of most non-chordate invertebrates in its position, form, and development. The chordate nerve cord is positioned dorsal to the gut tube, while the nerve cord of most non-chordates is found ventral to the digestive tract. The chordate nerve cord is also hollow and fluid-filled, while that of non-chordates is typically solid. Lastly, the chordate nerve cord forms by a invagination, an embryonic process in which neural tissue gathers dorsally on the outer surface of the embryo, folds into a tube, and then sinks inward (invaginates) to take up its internal position. The nerve cords of non-chordates do not form by invagination, instead cells typically move inward to their internal positions individually.

The pharyngeal slits are paired longitudinal series of perforations through the wall of the pharynx (the region of the digestive tract between the mouth and esophagus). These slits appear in in the embryo stage of all chordates and may persist into adulthood or may close and disappear before birth or hatching (as is the case in mammals, for example). Pharyngeal slits allow water that is brought in via the mouth to exit without passing through the entire digestive tract. In ancestral chordates and modern urochordates and cephalochordates (as well as hemichordates) pharyngeal slits function as suspensio-feeding structures. Vertebrates have modified the slits and their support structures for other functions including gas exchange, jaw support, and hearing.

The post-anal tail is a muscular region of the body that extends beyond the anus. In non-chordates the digestive tract usually extends the entire length of the body and the anus is typically positioned at the posterior end of the body (an earthworm is a good example of this type of body plan). The chordate post-anal tail includes skeletal support and musculature that improves the locomotion of many aquatic chordate species.

Chordate characteristics All chordates possess the four trademarks of the phylum: a notochord; a dorsal, hollow nerve cord; pharyngeal slits; and a muscular, postanal tail. (From Campbell 6th edition's Fig 34-2.)

Notochord function The notochord resists shortening in length, but is flexible laterally. Without a notochord, lateral muscle contraction collapses the body along its length. With a notochord muscle contractions on alternating sides efficiently flex the body in simple swimming strokes. (From Kardong 3rd edition's Fig 2.5.)

Neurulation - development of the dorsal hollow nerve cord. The neural tube originates as a plate of dorsal ectoderm just above the developing notochord. This "neural plate" soon folds inward, rolling itself into the neural tube, which will become the central nervous system - the brain and spinal cord. These organs are hollow in most chordates because of this mechanism of development. (From Campbell 6th edition's Fig 47-11.)

Post-anal tail. Recall that the digestive tract of the shark terminates at the cloaca, between the pelvic fins. The remainder of the body then represents a post-anal tail and plays a huge role in propulsion during aquatic locomotion. The paired fins play little or no propulsive role in most fish. Seen in this photo is a Blue Shark (Prionace glauca) swimming underwater in the Pacific Ocean, thirty miles off of San Diego, California. Blue sharks have been recorded swimming at speeds in excess of 35 mph!

 

Examples:

The phylum Chordata includes three subphyla:

• Urochordata (tunicates or sea squirts)
• Cephalochordata (lancelets)
• Vertebrata (vertebrates)

Examples of each subphylum are shown at right. For more detailed information on each subphylum follow the links above.

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A larval tunicate - note the chordate characteristics in the diagrammatic view.

Metamorphosis of a larval urochordate. Note the loss of all chordate characteristics except the pharyngeal slits. Click image to enlarge.

Adult urochordates. Note the inhalant (larger) and exhalant siphons and the striking convergence with sea sponges -click here for an example.

Subphylum Urochordata: a tunicate. (a) An adult tunicate, or sea squirt, is a sessile animal commonly arranged in a U shape (photo is approximately life-sized). (b) In the adult, prominent pharyngeal slits function in suspension feeding, but the other chordate characteristics are not obvious. (c) In the tunicate larva, which is a free-swimming but nonfeeding "tadpole," the chordate characteristics are evident: It has a notochord, dorsal nerve tube, and a tail with muscle segments. (From Campbell 6th edition's Fig 34-3.)

Subphylum Cephalochordata: the lancelet Branchiostoma (also called amphioxus). This small invertebrate displays all four chordate characteristics. The pharyngeal slits function in suspension feeding. Water passes into the pharynx and through slits into the atrium, a chamber that vents to the outside via the atriopore. Food particles trapped by mucus are swept by cilia into the digestive tract. The muscle segments you can see in this photo of a lancelet produce the sinusoidal swimming of these animals. (From Campbell 6th edition's Fig 34-4a.)

Subphylum Cephalochordata: the lancelet Branchiostoma. The muscle segments you can see in this photo of a lancelet produce the sinusoidal swimming of these animals. (From Campbell 6th edition's Fig 34-4b.)

Subphylum Vertebrata (Class Mammalia): The Vertebrates are characterized by a well-developed brain, cranium and sense organs.  As their name implies, they are also characterized by vertebrae which protect their dorsal hollow nerve cord.  Bone and a muscular phayngeal pump are also characteristics of vertebrates.  The mammal seen here (Canis familiaris) is representative of the class Mammalia, displaying both of the class' defining characteristics - mammary glands and hair.