Q.Mention four fundamental characters of Chordata.

[!TLDR] The four fundamental chordate characters are the notochord, dorsal tubular nerve cord, pharyngeal gill slits, and post-anal tail.

[!ANSWER] The four fundamental characters of Chordata are: a notochord; a dorsal, tubular (hollow) nerve cord; paired pharyngeal gill slits; and a post-anal tail — present in every chordate at some stage of its life cycle.

[!ANSWER] Four structural features, taken together, define phylum Chordata.

The notochord is a flexible, rod-like structure providing longitudinal support along the back of the animal, present at least in the embryo of every chordate (and persisting into the adult in some groups, such as Cephalochordata). The dorsal, tubular nerve cord is the main nerve cord, running above the notochord and hollow in cross-section — unlike the solid, ventral nerve cord of non-chordates. Pharyngeal gill slits are paired openings connecting the pharynx to the exterior, present in every chordate at some stage of development, whether or not they persist as functional respiratory structures in the adult (as they do in fish, but not in reptiles, birds or mammals). The post-anal tail is a tail extending behind the anus, present at some point in every chordate's life, even if reduced to a small vestige in the adult of some groups.

Every member of phylum Chordata expresses this same combination of features at some stage of its individual development, even though the four features may look very different, or be greatly reduced, from one chordate group to another.

[!ANSWER] Chordata's four fundamental characters: notochord, dorsal tubular nerve cord, pharyngeal gill slits, and post-anal tail — each present at some stage of life in every chordate.

• Naming only two or three of the four characters (a common slip is forgetting the post-anal tail).
• Assuming all four characters must be visible in the adult — several groups (e.g. mammals, tunicates) show some of them only in the embryo or larva.