Hint: All life forms can be grouped into various groups (taxonomic rank) based on their shared characteristic. The classification begins with Domain; the organism can either belong to the prokaryotic group or with eukaryotes. The final taxonomic rank is the species rank. Individuals that belong to a particular species share very similar characteristics such as DNA sequence, morphology, etc.
A phylum is a taxonomic rank that includes many species. The organisms that belong to the same phylum may not show many common characteristics but have some that are very distinguishable. Animals that belong to the phylum Chordata are chordates. This phylum can be further differentiated into three subphyla- Vertebrata, Tunicata, and Cephalochordata.
Animals that belong to Vertebrates are more advanced than the ones that belong to the other two subphylums. All organisms, including vertebrates, belonging to this phylum show some common features such as the presence of a notochord. The notochord is a supportive cartilaginous structure. Chordates also possess a dorsal neural tube. Vertebrates have a more evolved form of this neural tube i.e. spine. Some chordates also have pharyngeal slits.
Cephalochordates (for example lancelets) and tunicates (such as sea squirts) are small animals found in marine ecosystems. Vertebrates (fishes, amphibians, mammals, reptiles, and birds) range from quite small fishes to really large mammals. The major difference between other chordates and vertebrates is the presence of a vertebral column. This vertebra has evolved from the notochord itself.
Note: Organisms that belong to the phylum Hemichordata have some features that resemble chordates. They have stomochords instead of notochords. However, the composition of both structures is quite similar. They also possess a dorsal neural tube as well as a smaller ventral nerve cord.
Other organisms that are a close relative of chordates are the ones that belong to the phylum Echinodermata. All chordates exhibit bilateral symmetry. Echinoderms exhibit bilateral symmetry when they are larvae, as they grow into adulthood they show radial symmetry.