Answer: Notochord.
- A Radula
- B Notochord
- C Water vascular system
- D Cnidoblasts
Correct answer: B. Notochord
Explanation: The presence of a notochord at some stage of life is a defining feature of phylum Chordata, along with a dorsal hollow nerve cord and paired gill slits.
Moving across the animal phyla, body organisation becomes progressively more complex: from no symmetry and cellular organisation (Porifera) through radial/diploblastic (Cnidaria), bilateral/acoelomate (Platyhelminthes), pseudocoelomate (Nematoda), to a true coelom with full organ systems (Annelida and beyond).
Concept context
Phylum-level classification of the animal kingdom from Porifera to Chordata, based on body symmetry, coelom, and segmentation. One of the most example-heavy chapters for NEET.