What progressive features of chordates ensured their further evolution?

The Chordate type unites animals that are very diverse in appearance, lifestyle and habitat. Chordates are found in all major environments of life: in water, on the surface of land, in the soil and, finally, in the air.
The main features of the type are: 1) bilateral symmetry of the body; 2) through intestines; 3) internal axial skeleton-chord; 4) the neural tube that is located above the notochord; 5) a closed circulatory system; 6) gill slits of the pharynx as respiratory organs.