Q.Describe the general characteristic features of phylum Chordata that distinguish it from every non-chordate phylum discussed in this chapter.

Step 1. Notochord. A long, flexible, mesoderm-derived rod running the length of the body, dorsal to the gut, present in the chordate embryo (and often the adult); this feature alone is absent in every one of the ten non-chordate phyla studied earlier in the chapter, from Porifera through Hemichordata.

Step 2. Dorsal, hollow nerve cord. Chordates possess a single, tubular nerve cord running along the dorsal side of the body, above the notochord — a sharp contrast to the solid, ventral nerve cord found in non-chordates such as Annelida and Arthropoda.

Step 3. Paired pharyngeal gill slits. Openings connecting the pharynx to the exterior, used for respiration in aquatic chordates and retained, though non-functionally, as an embryonic feature even in land vertebrates; no non-chordate phylum possesses this feature.

Step 4. Post-anal tail. A tail extending behind the anal opening, present at some stage in nearly all chordates, again not found as such in the non-chordate phyla.

Step 5. Together, not separately. No single one of these four features alone is unique to Chordata (a rod-like support structure, for instance, is not itself remarkable), but the combination of all four, present together at some point in the life cycle even if only in the embryo, appears in no other phylum — this is why the four are always cited as a set. …

List and explain each of the four defining chordate features individually, then explain why it is their combination — not any single fea …

• Naming only the notochord and forgetting the other three features (dorsal hollow nerve cord, gill slits, post-anal tail) that together define the phylum. …