Chordate - Definition

Definition

1 = bulge in spinal cord ("brain") 2 = notochord 3 = dorsal nerve cord 4 = post-anal tail 5 = anus 6 = digestive canal 7 = circulatory system 8 = atriopore 9 = space above pharynx 10 = pharyngeal slit (gill) 11 = pharynx 12 = vestibule 13 = oral cirri 14 = mouth opening 15 = gonads (ovary / testicle) 16 = light sensor 17 = nerves 18 = metapleural fold 19 = hepatic caecum (liver-like sack) Anatomy of the cephalochordate Amphioxus. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.

Chordates form a phylum of creatures that are based on a bilateral body plan, and is defined by having at some stage in their lives all of the following:

• A notochord, in other words a fairly stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, and in wholly aquatic species this helps the animal to swim by flexing its tail.
• A dorsal neural tube. In fish and other vertebrates this develops into the spinal cord, the main communications trunk of the nervous system.
• Pharyngeal slits. The pharynx is the part of the throat immediately behind the mouth. In fish the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live.
• Post-anal tail. A muscular tail that extends backwards behind the anus.
• An endostyle. This is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus. It also stores iodine, and may be a precursor of the vertebrate thyroid gland.