Q.“All vertebrates are chordates but all chordates are not vertebrates”. Justify the statement.

This statement follows directly from what defines a chordate and what defines a vertebrate.

• A chordate is any animal that, at some stage of its life, possesses three features together: a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits.
• Phylum Chordata is divided into three subphyla — Urochordata, Cephalochordata and Vertebrata. The first two are grouped as protochordates and never develop a backbone.
• In vertebrates, the notochord is present only in the embryo and is later replaced by a vertebral column (backbone) of cartilage or bone.
• Because every vertebrate does possess a notochord (in the embryo) along with the other chordate features, every vertebrate satisfies the definition of a chordate. …

To understand this statement, we start from what places an animal in phylum Chordata. An animal is a chordate if, at some point in its life, it shows three characters together: a notochord (a firm, flexible supporting rod running along the body), a dorsal hollow nerve cord (the central nervous system lying along the back as a single tube), and paired pharyngeal gill slits (openings arranged in pairs through the wall of the pharynx). Any animal with this combination belongs to the chordates, whatever its outward appearance.

Phylum Chordata is then split into three subphyla — Urochordata (also called Tunicata), Cephalochordata and Vertebrata. Urochordata and Cephalochordata are together called protochordates, and neither of them ever develops a backbone.

• In Urochordata, the notochord is confined to the tail of the larva and disappears in the adult.
• In Cephalochordata, the notochord runs from head to tail and stays throughout life, but there is still no vertebral column.
• In Vertebrata, the notochord appears only during the embryonic stage and is later replaced by a vertebral column made of cartilage or bone.

Method: Set-and-subset (Venn diagram) reasoning

Treat "Chordata" and "Vertebrata" as two circles on a Venn diagram rather than as a list of shared and differing features, and reason about the circles' relationship directly.

Draw one large circle for Phylum Chordata: it contains every animal that shows, at some life stage, a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. Inside this large circle, draw three smaller circles for the three subphyla: Urochordata, Cephalochordata and Vertebrata.

First ask: is Vertebrata's circle inside the Chordata circle, or does it stick out on its own? Since Vertebrata is explicitly one of the three subphyla INTO WHICH Chordata is divided, its circle sits entirely inside the larger Chordata circle — every point (every vertebrate) is automatically also a point inside Chordata. This directly gives "all vertebrates are chordates," because Vertebrata is a subset of Chordata by definition, not by coincidence. …