Q."All vertebrates are chordates but all chordates are not vertebrates." Justify the statement with examples.

A common mistake is to treat "chordate" and "vertebrate" as the same word, as though every animal with the chordate features must also have a backbone. Sorting out the difference is one of the clearest logical points in the whole chapter, and it rests on knowing what makes a chordate in the first place.

An animal is a chordate if it shows three features together, at some point in its life:

• a notochord, a firm, flexible rod running along the body that provides internal support;
• a dorsal, hollow nerve cord, meaning the central nervous system lies along the back, is tube-like and forms a single cord; and
• paired pharyngeal gill slits, openings arranged in pairs in the wall of the pharynx.

Alongside these, chordates are bilaterally symmetrical, triploblastic and coelomate, work at the organ-system level, and typically have a post-anal tail and a closed circulatory system.

Now the distinction. Phylum Chordata is divided into three subphyla — Urochordata, Cephalochordata and Vertebrata. The first two are grouped together as protochordates and never develop a backbone: in urochordates the notochord is confined to the tail of the larva and is lost in the adult, while in cephalochordates it runs from head to tail throughout life. Only in the Vertebrata does the notochord, present in the embryo, get replaced in the adult by a vertebral column made of cartilage or bone.

This leads to the key statement: every vertebrate is a chordate, but not every chordate is a vertebrate. The protochordates are chordates that simply never grow a backbone. …