4.1. Chordates

A major distinctive feature of chordates in relation to other animals is the ability of producing three closely-related hard tissues: cartilage, bone and teeth. Most of the fossil record available for the group derives from these hard tissues which improved the defense, locomotion and feeding abilities of these animals.

The diagnostic feature of a chordate is having a notochord, which is a flexible rod made out of a material similar to cartilage. If a species has a notochord, it is a chordate. The notochord lies along the anteroposterior ("head to tail") axis. It is usually found closer to the dorsal than to the ventral surface of the animal, and it is composed of cells derived from the mesoderm. Many functions have been attributed to the notochord. These include serving as a site of muscle attachment, a vertebral precursor, and a midline tissue that provides signals to the surrounding tissues during development.

Notochords are thought to be advantageous (both in an evolutionary and developmental context) because they provide(d) rigid structure for muscle attachment, but were still flexible. In some chordates, it persists throughout life as the main axial support of the body, while in most tetrapods it becomes the nucleus pulposus of the intervertebral disc. The notochord plays a key role in signalling and coordinating development. Embryos of vertebrates still form transient notochord structures today during the gastrulation phase of development. The notochord is found ventral to the neural tube.

The phylum Chordata is formed by three major groups: Cephalochordata (lancelets), Tunicata (sea squirts) and Craniata (vertebrates).

Lancelets

Lancets make the oldest of the three. They comprise about 32 species of fish-like marine chordates in the order Amphioxiformes, with a global distribution in shallow temperate (as far north as Scotland) and tropical seas, usually found half-buried in sand. They are the modern representatives of the subphylum Cephalochordata. In Asia, they are harvested commercially as food for humans and domesticated animals. Although lancelets split from vertebrates more than 520 million years ago, their genomes hold clues about evolution, particularly how vertebrates have employed old genes for new functions.

Lancelets are typically 5 cm (2.0 in) long, or 7 cm (2.8 in) at the longest. They have a translucent, somewhat fish-like body, but without any paired fins or other limbs. A relatively poorly developed tail fin is present, so they are not especially good swimmers. While they do possess some cartilage-like material stiffening the gill slits, mouth, and tail, they have no true skeleton.

Lancelets also have oral cirri, thin tentacle-like strands that hang in front of the mouth and act as sensory devices and as a filter for the water passing into the body. Water passes from the mouth into the large pharynx, which is lined by numerous gill-slits. The ventral surface of the pharynx contains a groove, called the endostyle, which, connected to a structure known as Hatschek's pit, produces a film of mucus. Ciliary action pushes the mucus in a film over the surface of the gill slits, trapping suspended food particles as it does so. The mucus is collected in a second, dorsal, groove, and passed back to the rest of the digestive tract. Having passed through the gill slits, the water enters an atrium surrounding the pharynx, then exits the body via the atriopore.

Both adults and larvae exhibit a "cough" reflex to clear the mouth or throat of debris or items too large to swallow. In larvae the action is mediated by the pharyngial muscles while in the adult animal it is accomplished by atrial contraction.

The remainder of the digestive system consists of a simple tube running from the pharynx to the anus. The hepatic caecum, a single blind-ending caecum, branches off from the underside of the gut, with a lining able to phagocytize the food particles, a feature not found in vertebrates. Although it performs many functions of a liver, it is not considered a true liver but a homolog of the vertebrate liver.

Lancelets have no respiratory system, breathing solely through their skin, which consists of a simple epithelium. Despite the name, little if any respiration occurs in the gill slits, which are solely devoted to feeding. The circulatory system does resemble that of primitive fish in its general layout, but is much simpler, and does not include a heart. There are no blood cells, and no haemoglobin.

Sea squirts

Tunicates live as solitary individuals, but others replicate by budding and become colonies. They are marine filter feeders with a water-filled, sac-like body structure and two tubular openings, known as siphons, through which they draw in and expel water. During their respiration and feeding, they take in water through the incurrent (or inhalant) siphon and expel the filtered water through the excurrent (or exhalant) siphon. Most adult tunicates are sessile, and are permanently attached to rocks or other hard surfaces on the ocean floor. Others, such as salps, doliolids and pyrosomes, swim in the pelagic zone of the sea as adults. Various species are commonly known as sea squirts, sea pork, sea livers, or sea tulips.

The earliest species of tunicate appeared in the fossil record in the early Cambrian period. Despite their simple appearance and very different adult form, their close relationship to the vertebrates is evidenced by the fact that during their mobile larval stage, they possess a notochord or stiffening rod and resemble a tadpole. Their name derives from their unique outer covering or "tunic", which is formed from proteins and carbohydrates, and acts as an exoskeleton. In some species, it is thin, translucent, and gelatinous, while in others it is thick, tough, and stiff.

Summary

Chordates are animals that have a notocord. This includes cephalochordates (lancelets), urochordates (sea-squirts) and all vertebrates. The body of a lancelet resembles that of a fish and it has a notocord but lacks a skeleton. Sea-squirts are mostly marine filter-feeders with a sessile adult form that lives attached to the bottom, but a free-swimming larva that presents a notocord.

Key terms

Chordate, urochordate, cephalochordate, lancelet, amphioxus, sea-squirt, tunicate, skeleton, notocord

Figure credits

Figure 1 by Mokele at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=9004707

Figure 2 by © Hans Hillewaert, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=5712836

Figure 3 by Jon Houseman - Jon Houseman and Matthew Ford, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=33180059

Figure 4 by Piotr Michał Jaworski; PioM EN DE PL - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4481198

Figure 5 by Silke Baron - originally posted to Flickr as Sea Squirts, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=9971394

Figure 6 by Jon Houseman - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=25858232