Chordates are members of a phylum of deuterostome triploblast characterised by the presence of a notochord.
This rigid structure in the dorsal area is located between the digestive tube, on the ventral side, and the neural tube, on the dorsal side. It constitutes an endoskeleton, which is replaced in vertebrates, by the spine.
The three other characteristics of chordates are:
- A neural tube located on the dorsal side (epineurian), while protostomes are hyponeurians: their nervous system is located on the ventral side;
- Gill slits near the pharynx;
- A tail beyond the anus.
Of course, some of these elements can disappear over the course of evolution or during embryonic development.
Chordates are subdivided into two sub-subphyla: prochordates, with cephalochordates and urochordates, and vertebrates.
This ascidian is a urochordate that lost its cord over the course of its embryonic development. © sarsifa CC by-nc-sa 2.0